Ash caught Swellow as a Taillow in the episode You Never Can Taillow!. It was the leader of a flock of wild Taillow. It stole Max's chocolate and challenged Pikachu to a battle. It also led the entire flock on a ceaseless attack until Brock and his Forretress intervened and used Explosion to drive them away. Taillow then showed up alone and challenged Pikachu, and even though it took countless Thunderbolts and a Thunder from Pikachu, it still kept fighting. On Brock's advice, Ash captured Taillow to prevent it from getting hurt anymore. Ash then sent it out and Brock helped it recover.
Its first battle was against Nicholai and his Zigzagoon in In the Knicker of Time!. Taillow's impressive flying techniques allowed it to match Nicholai's newly caught Pokémon well. The winner of the battle is unknown. It also fought against Anthony's Pelipper in You Said a Mouthful!, where it handled the battle well but lost when a Magby hidden inside Pelipper's bill used Flamethrower.
In Brave the Wave, Ash chose it to battle the Fighting-type Gym Leader Brawly's Machop. Taillow managed to land several Wing Attacks on Machop, but Brawly's Pokémon managed to weather the attacks to land a devastating Cross Chop at the vital moment, taking Taillow out in a single hit. Taillow, with Corphish, Treecko and Pikachu, took up a meditation program to prepare for the rematch. However, Taillow was substituted for Corphish in the rematch.
In Pros and Con Artists, Ash used Taillow to battle Drew and his Roselia, as shown in a flashback later in the episode Ash was sure Taillow could beat Roselia due to Taillow having a type advantage. Taillow used Peck which hit Roselia, whom quickly counterattacked with Magical Leaf, but Taillow swiftly dodged it, Roselia used Petal Dance which Taillow dodged by flying upwards, then it went for a Peck but Roselia used Stun Spore, Taillow attempted to dodge by flying upwards, but it was hit by the Stun Spore anyways, paralyzing it. May then arrived at what Drew called the best part, Roselia used Solar Beam, and due to the paralysis, Taillow was unable to dodge causing the Solar Beam to hit Taillow knocking it out.
In That's Just Swellow, Ash entered Taillow in a PokéRinger tournament. Unbelievable determination allowed Taillow to shrug off Electric-type attacks that reigning champion Volt's Volbeat was using, and it made a counterattack to fly to victory. In the final, Taillow went up against James and Dustox, and started strongly until a sand tornado kicked up. Determined to win, Taillow flew into the storm, but was too small against the wind. Desperate, it evolved into Swellow and easily broke through, using Wing Attack to flick the ring onto the goal and win the tournament. It then single handedly sent the sore losers Team Rocket blasting off.
As a newly evolved Swellow, winning the PokéRinger tournament
Several episodes later, in Sky High Gym Battle!, Swellow fought Winona's Shiny Swellow despite Winona's using more powerful attacks such as Hyper Beam and Aerial Ace. However, Ash quickly recovered from Hyper Beam and ordered Swellow to create a dust cloud to blind and slow Winona's Swellow down. It then struck with Wing Attack and knocked out Winona's Swellow, winning the battle for Ash.
After this victory, Ash became determined to teach his Swellow Aerial Ace, beginning in Pokéblock, Stock, and Berry. Swellow initially struggled but with the help of Vladimir and his Pidgeotto, Ash was able to teach Swellow the move in Lessons in Lilycove!.
In Solid as a Solrock it teamed up with Pikachu in a Double Battle against Tate and Liza, as Ash inspected the terrain and decided an aerial fighter would be necessary. The two of them struggled to defeat Lunatone and Solrock, with Swellow only landing a Peck on Lunatone with little damage inflicted. Swellow came to Pikachu's rescue, bearing it into the air and away from the plummeting it was taking. Pikachu's Thunderbolt was sent back onto the team by Lunatone's Light Screen, but Swellow managed to shrug off most of the damage. Swellow then used a newly learned Double Team to distract the duo while it flew above them. Pikachu used Thunder on some thunder clouds above, the clouds spat the electricity back at Pikachu and Swellow, and what should hurt Swellow, powered it up along with Pikachu and turned Swellow a metallic golden color. This was called Thunder Armor. Swellow easily diverted Solar Beam and Ice Beam and then took Lunatone out with a supercharged Aerial Ace.
In Eight Ain't Enough, Ash used Swellow in the second round of his Sootopolis Gym battle, it went up against Juan's Whiscash, Swellow used Quick Attack but was caught by Whiscash's barbel's, Whiscash then used Tickle, which weakened Swellow's Defense, Whiscash then launched a Hyper Beam which Swellow was unable to dodge due to still being caught in Whiscash's barbel's, much to Juan's surprise, Swellow got up and was now free from the barbel's, Swellow launched Aerial Ace and Juan had Whiscash go underwater, but much to his surprise Swellow was able to dive underwater and hit Whiscash with the attack. Whiscash launched another Hyper Beam which Swellow dodged using its Quick Attack, it then transitioned into Aerial Ace. Whiscash attempted to dodge by diving underwater but Swellow was faster, hitting Whiscash, knocking it out. Juan then used his last and most powerful Pokémon, Milotic. Milotic used Hydro Pump, which Swellow was able to dodge and was able to land an Aerial Ace, but it didn't do much damage to the Tender Pokémon, Milotic used Iron Tail which Swellow dodged by using Double Team, but Milotic used Twister, destroying the copies and crashing Swellow into the roof, knocking Swellow out.
In A Judgment Brawl, Ash used Swellow as his fifth Pokémon in his Full Battle against Katie in the Victory Tournament part of the Ever Grande Conference, it went up against Katie's Venomoth, since it had a Type Advantage against it. Venomoth used Stun Spore, but Swellow was able to dodge and it successfully used Peck on Venomoth, but Venomoth countered with Disable, disabling Swellow's Peck for a while, but Ash wasn't worried, as it had more Flying-type moves than Peck and it used Aerial Ace, making a direct hit. Venomoth countered with Supersonic, but Swellow used Quick Attack, dodging Supersonic and hitting Venomoth, knocking it out. It then went up against her Scizor. Both Pokémon used Quick Attack and clashed multiple times but neither was able to gain an advantage over the other, Swellow used Peck but this was countered by Scizor's Double Team who then dodged. Scizor with the Double Team copies, used Metal Claw and Swellow countered with a Double Team of its own and then dodged. It then used Aerial Ace with the Double Team copies, but Scizor transitioned into Hidden Power and destroyed the copies, but the a Swellow was able to dodge and use Quick Attack, Scizor then used Metal Claw but that Swellow was just a copy and the real Swellow used its Aerial Ace which was sped up by Quick Attack, successfully hitting Scizor, knocking it out. Katie then sent out her last Pokémon, her Walrein. Swellow used Quick Attack but it did nothing to Walrein and was deflected. Walrein used Ice Beam and although it didn't make a direct hit, it hit one of Swellow's wings. Swellow used Peck but Walrein dodged by going underwater, as it was a water field. Walrein then came from behind and used Ice Beam, knocking Swellow out.
In Choose It or Lose It!, Ash used Swellow as his fifth Pokémon in his Full Battle against Morrison in the Victory Tournament, it went up against Morrison's Gligar. Gligar started out with Guillotine, which was a one-hit knockout move, but Swellow used Double Team to avoid the attack. It used Peck but was dodged and Gligar used Iron Tail but Swellow dodged with Quick Attack, Gligar used Hidden Power instead, hitting Swellow, but it was able to recover using Aerial Ace, while Gligar used Steel Wing, both of them taking the attack. They both used the same attacks again dealing major damage. They used the same attacks a third time, with the collison ending their battle in a tie.
Catching Donphan at the last second
In At the End of the Fray, Ash used Swellow as his fourth Pokémon in his Full Battle against Tyson in the Victory Tournament, it went up against his powerful Hariyama, who had already defeated Torkoal and Corphish. Swellow used Peck, which the Arm Thrust Pokémon was able to catch in its hands, it then used Seismic Toss, throwing Swellow into a rock, but Swellow was now free, Hariyama used Focus Punch but Swellow used Aerial Ace, allowing it to avoid Focus Punch and hit Hariyama, which was super-effective, knocking it out. It then went up against Tyson's Donphan, Donphan used Sandstorm, causing Ash to be unable to see Swellow, Swellow flapped its wings, removing the Sandstorm, but this gave Donphan the opportunity to use Rollout, using a rock to hit Swellow, knocking it to the ground. Donphan used multiple Rollout attacks hitting Swellow each time and getting more powerful with each Rollout attack. Donphan launched another Rollout, intending to finish Swellow off as it lay on the ground, but at the last second it was able to catch Donphan using its legs. Swellow lifted off and then threw the Armor Pokémon into a rock, knocking it out. It then went up against Tyson's Metagross, the Iron Leg Pokémon used Psychic, dealing damage and trapping Swellow in the air, with it unable to break free. However Swellow was able to break free with Quick Attack, but Metagross's high defense allowed it to deflect the attack, causing Swellow to take damage, Metagross used Hyper Beam, knocking Swellow out. However, Swellow was able to make a crack in Metagross's body during the battle that would later be used by Grovyle and Pikachu, the latter managing to defeat it.
Swellow went with Ash through the Kanto Battle Frontier and fought well against Frontier Brains. In Tactics Theatrics!!, it teamed up with Corphish to battle Dome Ace Tucker and his Swampert and Arcanine. Swellow launches a Quick Attack on Swampert but Arcanine leaps forward and collides with Swellow, knocking it away from its intended target. It kept trying to attack Swampert, but Arcanine's speed was too great and it continued to block it. Swellow managed to dodge Flamethrower with Double Team, but Arcanine powered up into a Fire Blast and hit Swellow dead on. After some maneuvering from Arcanine and Swampert, Swellow and Corphish were trapped inside a fire and water structure. Swellow managed to smash Arcanine into Swampert for a direct hit on both, while Corphish protected Swellow from Fire Blast with Bubble Beam. The smoke from the clash between bubbles and fire blinded both of Tucker's Pokémon and Swellow managed to hit Swampert in its blind spot. Swampert fainted just as Corphish finished off Arcanine, winning Ash the Tactics Symbol.
In Ka Boom with a View!, Swellow went up against Spenser's Venusaur, who had just flattened Ash's Heracross. Changing tactics, Ash ordered Swellow to fly towards the ocean. While flying, it could easily dodge Venusaur's Vine Whip and Razor Leaf. On the beach, battle recommenced with Swellow dodging all of Venusaur's attacks with Double Team and striking hard with Quick Attack. A final Aerial Ace took down Venusaur and Spenser replaced it with Claydol. Swellow was noticeably worried but fought on anyway with Quick Attack. However, Claydol dodged with Teleport and then knocked Swellow out of the air with Rapid Spin. A follow-up Hyper Beam knocked Swellow out of the match.
In Pokémon Ranger - Deoxys' Crisis! (Part 2), it did its best in a fight against Deoxys, but could not possibly match the Speed Forme and was knocked to the ground.
In Slaking Kong, Swellow took on a giant robot Slaking long enough for Sceptile to rescue May. It then slammed it into the ground to permit others to attack it. In Aipom and Circumstance, it took on a whole flock of mechanical Fearow despite being outnumbered and managed to cause serious damage.
Ash left Swellow at Professor Oak's lab when he set off for Sinnoh in Home is Where the Start Is!
In The Semi-Final Frontier!, Swellow was brought back to Ash's team where it battled against Tobias in the semi-finals of the Lily of the Valley Conference. It used Quick Attack on Tobias's Latios; however, Latios was able to dodge it. Swellow then used Aerial Ace and flew from the sky towards Latios, coming down with great force. However, Latios used Luster Purge on Swellow, knocking it out.
It was mentioned in The Fires of a Red-Hot Reunion! by Professor Oak, who mentioned that Swellow would sometimes have air races with Staraptor at the lab, and Unfezant would make a good addition to those races.
Swellow was later reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed in a group photo with him.