Treecko was caught in the episode Tree's a Crowd after Ash helped it in trying to save its old tree home. Soon afterward, it had a battle with Ash's Pikachu, and Ash caught it as a result. In the next episode, Treecko struggled to make friends with the other Pokémon until it rescued May's Torchic from a wild Seviper, though it was badly hurt in the process. Later in the episode, Treecko went off to do some special training on its own; when it had perfected a new variation of Pound in which it spun in the air before hitting the target, it battled the Seviper again and won.
In The Winner by a Nosepass, Ash choose Treecko to battle Roxanne's Geodude where it quickly lost. Brock pointed out that even though Grass-types are strong against Rock-types, Treecko did not have an advantage due to it not knowing any Grass-type moves. In Brave the Wave it was used during Ash's Gym Battle against Brawly. While it managed to defeat Machop, it was still beaten by Brawly's Hariyama, causing Ash to lose the battle. In Turning Over a Nuzleaf, when it and the other Pokémon were separated from their Trainers, Treecko frequently taunted Corphish, though they later made amends in order to beat Team Rocket. Treecko battled Brawly's Hariyama again in Just One of the Geysers and won, earning Ash the Knuckle Badge.
Treecko battled a Shiftry in Jump for Joy! to rescue a Nurse Joy. Ash revealed in What You Seed is What You Get that the Shiftry had inspired him to try to teach Treecko how to use Bullet Seed. This became a reality when they met Natasha and her Grovyle, who helped Treecko learn the move using watermelon seeds.
In Going, Going, Yawn! Ash used Treecko as his second Pokémon in his Lavaridge Gym battle. It went up against Flannery's Slugma depsite a type disadvantage. After using its speed to dodge Slugma's Flamethrower, Treecko used Quick Attack but the damage was halved thanks to Magcargo's Reflect earlier in the Gym Battle. After dodging another Flamethrower, Treecko used Pound which was also weakened by Reflect. Slugma then used Smog, hiding itself. Treecko used multiple Bullet Seed attacks but they all missed. Treecko was then hit by Slugma's Body Slam. It used Bullet Seed to escape but was burned by Flamethrower whilst escaping. Ash then recalled Treecko. Ash later sent out Treecko to battle Flannery's final Pokémon, Torkoal, despite being still injured from the burn. Torkoal ended up defeating it with a powerful Overheat.
In Gulpin it Down it ended up temporarily becoming giant alongside a wild Gulpin that had been terrorizing a large city.
In Exploud and Clear!, whilst the other Pokémon ate food, Treecko didn't eat and acted distant, hiding up a tree and ignoring attempts by Ash and Corphish to be friendly. This worried Max but Ash assumed that it was just in a bad mood. All of a sudden, a Loudred started rampaging through the area, which caught Treecko's interest. The Loudred's rampage was stopped by its Trainer Guy. Guy mentioned his Loudred had been acting weird lately, which Ash compared to his Treecko. Ash decided to battle Guy and his Loudred as he thought exercise could solve its problems and Treecko volunteered itself to battle. After battling hard both Pokémon evolved at the same time, Loudred into Exploud and Treecko into Grovyle. Grovyle demonstrated its new Leaf Blade attack, putting a twig in its mouth. Ash and Guy decided to continue the battle, but Exploud ignored its trainer's commands, rampaging off. The two then followed Exploud. Whilst looking for Exploud, Grovyle refused to return to its Poké Ball and went to higher ground, which caused May to comment that Grovyle was more stubborn after evolving. After Team Rocket angered Exploud, Grovyle heard Exploud and followed it to a waterfall, battling it again. Ash and the others eventually caught up with Grovyle and Exploud with Ash and Guy deciding to pick up the battle, with Exploud ignoring Guy's commands again. Team Rocket then tried to capture Exploud again but Guy was able to protect it, earning its respect. With Exploud obeying Guy again, Ash decided to finish the battle using Grovyle again. The outcome of the battle was unknown.
In Go Go Ludicolo!, Grovyle was sent out with Ash's other Pokémon when Ash was deciding which three Pokémon he would use for his next Gym Battle. Corphish was immediately jealous of it, noticing that it had recently evolved. It was used a lot by Ash, such as getting oranges from a tree, freeing Ash's Pokémon from a net and defeating Team Rocket, getting lots of praise in the process from the others, all of which made Corphish more jealous and angry. Ash was tempted to use Grovyle to battle Poncho and his Ludicolo but used Corphish instead, after it got angry again and wanted to battle as well.
In Balance of Power!, Ash used Grovyle as his third and final Pokémon in his Petalburg Gym Battle against Norman, going up against Norman's final Pokémon, his Slaking. Both Pokémon stared off against each other initially with May mentioning that both Pokémon were big and scary. Grovyle used Bullet Seed which Slaking blocked with its hands, taking no damage. Norman mentioned that since Slaking was the Lazy Pokémon, weak attacks won't arouse interest. Grovyle launched a Leaf Blade attack, which Slaking caught and threw Grovyle. Slaking used Focus Punch, which Grovyle responded to with Bullet Seed, causing the Focus Punch to fail as Slaking lost its focus. However, this just made Slaking angry. Slaking launched a powerful Earthquake, which had the indirect effect of causing Team Rocket to be attacked by Dugtrio. Ash had Grovyle jump and use Pound but it got caught again by Slaking, who sent Grovyle flying and causing it to hit the roof. Grovyle used Bullet Seed, but Slaking launched a Hyper Beam that overpowered Bullet Seed, dealing major damage. Grovyle got up but looked weak. Norman suggested Ash forfeit for Grovyle's safety but Grovyle convinced Ash to continue. Grovyle's Overgrow ability immediately activated. Since it was weak, the power of its Grass-type moves increased significantly. Norman commented that the real battle was beginning. Slaking used Earthquake but Grovyle jumped and then jumped off Slaking when it tried to catch it. Slaking then used Hyper Beam but Grovyle jumped off a wall to dodge. It hit Slaking with a powerful Pound, but Slaking didn't look hurt at all. Slaking launched a full power Focus Punch, with Grovyle launching a full power Leaf Blade. Although it struggled to remain standing up afterwards, only Grovyle remained standing, defeating Slaking and earning Ash the Balance Badge.
In Sky High Gym Battle!, Grovyle battled Winona's Altaria and won, despite the type disadvantage. It was later used against Winona's Swellow, though it lost. In Vanity Affair it once again battled an Altaria, this time belonging to the Hoenn Elite Four's Drake. With its incredible jumping ability, Grovyle was able to attack Altaria while it was high up, but was unable to defeat it.
In The Great Eight Fate!, Ash used Grovyle as his first Pokémon in the second round of his Sootopolis Gym battle, going up against Juan's Luvdisc. Grovyle used Bullet Seed which Luvdisc dodged. Luvdisc countered with Water Pulse, which Grovyle was able to dodge by jumping. It went for a Leaf Blade but Luvdisc dodged and countered with Sweet Kiss. In the next episode, it was revealed that the Sweet Kiss confused Grovyle, causing it to be initially unresponsive and eventually bash its head against a wall. Although Brock told Ash to recall Grovyle, Juan took the initiative and had Luvdisc use Water Gun, knocking Grovyle out.
Grovyle was used many times in the Ever Grande Conference. Its first battle was in Shocks and Bonds where it teamed up with Glalie to battle Quilava and Charizard. Though it was able to defeat Quilava, it lost to Charizard, leaving Glalie to battle on its own.
In A Judgment Brawl, Ash used Grovyle as his sixth and last Pokémon in his Full Battle against Katie in the Victory Tournament, going up against Katie's last Pokémon Walrein. Walrein used Ice Beam and Grovyle was able to dodge and use Leaf Blade. However, Walrein dodged by diving underwater in the water field and coming back to hit Grovyle with Ice Beam from behind. Walrein confused Grovyle by diving and reappearing in different locations. To counter, Ash had Grovyle use Leaf Blade on the water, causing Walrein to emerge and take damage. Walrein used Ice Beam but Grovyle dodged and the Ice Beam froze the water, turning the field into ice. This was Ash's plan all along, preventing Walrein from going underwater. Grovyle then successfully used Quick Attack on Walrein crashing it into a wall, but Walrein wasn't finished and used Mimic, copying Grovyle's Quick Attack. Grovyle hit with Leaf Blade and Walrein used Body Slam, but Grovyle dodged and used Bullet Seed on Walrein. It then hit Walrein with a Quick Attack while dodging its Ice Beams. Walrein used Quick Attack and Ash had Grovyle move up close, surprising everyone since Walrein was larger. It dodged at the last second and used Leaf Blade, defeating Walrein and moving Ash to the next round of the Victory Tournament.
In Choose It or Lose It!, Ash uses Grovyle as his fourth Pokémon in his Full Battle against Morrison in the Victory Tournament, Grovyle went up against Steelix, who had already defeated Pikachu and Torkoal. Grovyle dodged Iron Tail and used Bullet Seed, which Steelix was able to dodge with Dig. Grovyle was able to jump from the incoming Dig and Steelix used Dragon Breath, but Grovyle was able to push through Dragon Breath with Leaf Blade, hitting it and defeating Steelix. Morrison then sent out his Gligar to match his speed. Grovyle used Quick Attack and Gligar was able to dodge. Gligar used Steel Wing on Grovyle but it jumped into the grass, hiding itself. Grovyle hit it with a Bullet Seed and Gligar used Steel Wing again. Although Grovyle hid in the grass again, Gligar used Steel Wing to try and search it out, but Grovyle was able to jump onto Gligar's back and attempted to use Leaf Blade, but Gligar shook it off and used Guillotine, Grovyle was unable to dodge, it attempted but use Bullet Seed, but Gligar dodged and hit it with the attack, the one-hit knockout move defeating it.
In At the End of the Fray, Ash uses Grovyle as his fifth Pokémon in his Full Battle against Tyson in the Victory Tournament, Grovyle went up against Metagross, a crack in Metagross's armor caused by Swellow gave Ash an idea on how to defeat it. Grovyle used its Bullet Seed attacks on the chink in the armor, however this didn't defeat Metagross and it used Hyper Beam, which Grovyle dodged and made the Iron Leg Pokémon unable to move allowing Grovyle to focus more Bullet Seed attacks on the crack, dealing major damage, Ash attempted to finish Metagross off with Leaf Blade, but Metagross used Meteor Mash, defeating Grovyle.
Grovyle battled Greta and her Hariyama in Wheel of Frontier, but was defeated.
In Odd Pokémon Out!, it developed a crush on a Meganium owned by the Nurse Joy from Camomile Island, and evolved into a Sceptile to rescue it when it had fallen into Team Rocket's hands. However, the Meganium was attracted to a Tropius, leaving Sceptile heartbroken and unable to use any moves. It regained its ability to use attacks in Cutting The Ties That Bind! to save Ash from Team Rocket with the use of Leaf Blade. It then used Quick Attack to save him and Bullet Seed to soften the fall. It was used in Ash's battle against Spenser in the following episode, Ka Boom with A View!, where it won Ash the Spirit Symbol after learning a new move, Solar Beam.
In Pokémon Ranger - Deoxys' Crisis! Parts One and Two, Sceptile helped battle a wild Deoxys. It later battled a Legendary Pokémon in Battling the Enemy Within!, Brandon's Regirock, when Ash was possessed by the evil spirit of the King of Pokélantis.
In Once More With Reeling!, it was used in the Battle Round of the Terracotta Town Pokémon Contest, where it went up against May's newly evolved Blaziken. The battle resulted in a tie where both May and Ash won the Ribbon.
Ash left Sceptile at Professor Oak's lab before setting off to Sinnoh.
In The Semi-Final Frontier!, Ash's Sceptile had its third battle against a legendary Pokémon when it was sent out against Tobias's Darkrai in the semi-finals of the Lily of the Valley Conference. It used its newly-learned attack, Leaf Storm, to attack Darkrai, but was hit by its Ice Beam first. Soon, Sceptile was hit by a Dark Void and was put to sleep, but was able to wake up almost instantly, shocking Tobias, and defeated Darkrai with a powerful Leaf Blade. Sceptile was the only Pokémon in the Sinnoh League (or the region for that matter) to defeat Darkrai. However, after all the damage it had taken it was unable to defeat Tobias's next Pokémon, Latios, whose Giga Impact had overpowered Leaf Blade, knocking Sceptile out.
Sceptile was reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed in a group photo with him.