Black started off with Bayleef, who was a Chikorita at the time. He stole it from Professor Elm's lab as he thought that Chikorita was strong. Following the phone call Gold got from Professor Elm when he reported the situation, Chikorita fired a Razor Leaf at him and battled Gold's Totodile. Chikorita had the upper hand due to the type-advantage but Pidgey stepped in and finished Chikorita off, resulting in Chikorita's defeat. After the battle, it left along with its Trainer.
Sometime before Gold arrived in Union Cave, Chikorita evolved into a Bayleef. Black sent out Bayleef during a rematch with Gold. Gold was surprised to see that Chikorita evolved which amused Black. Gold had Totodile battle Bayleef. The battle was tougher than last time as Bayleef dodged most of Totodile's attacks. As Gold used a strategy for Totodile to aim its Water Gun at Bayleef's feet, a Lapras emerged from underground. Black had Bayleef finish Lapras while Gastly paralyzed it with Mean Look. However, Lapras struck back and Bayleef was injured. Seeing that Bayleef was defeated and Lapras was unable to be caught, Black left the cave and Bayleef behind. Gold decided to take in Bayleef as his own.
During the Gym Battle with Bugsy, Bayleef had trouble listening to its new Trainer's commands, especially when it was recently taken in by Gold and it faced a Pokémon with a double type-advantage, which it had trouble dealing with. However, Bayleef managed to defeat it in the end.
Later, Gold sends it out to battle Sudowoodo when it was a giant Pokémon. Bayleef battles Black's Haunter after dealing with Team Rocket. As Haunter was weakened by the soundwaves coming from Team Rocket's machine, Bayleef managed to defeat it easily.
While under Black's ownership, Bayleef, as a Chikorita, was very rough and it matched its Trainer very well. It continued to be that way when it evolved until Lapras took it out and Black left it behind. It didn't trust Gold, its new Trainer, at first but with some support, Bayleef and Gold became in sync with each other.