Butterfree was caught by Ash as a Caterpie when it had lowered its guard down. It evolved into Metapod and then into Butterfree soon, after it witnessed Misty belittling Ash during Ash and Misty's 'battle'. It scattered Sleep Powder over the area upon Ash's order.
Ash sent out Butterfree to battle against a wild Onix. It used Psychic, which inflicted damage and prevented Onix from nearing them. Its attack left Onix confused and as Onix began thrashing wildly, the tunnel started to cave in and it was soon recalled.
Butterfree was used by Ash to battle against Haunter along with other Pokémon to rescue Sabrina, but their combined efforts were proven ineffective against it as it was quickly defeated by Haunter and recalled back to its Poké Ball.