Pokemon Season 1 Episode 31 - Dig Those Diglett!

⁣Ash finds himself among a group of Pokémon Trainers hired to protect a
construction site from interfering ground Pokémon. Jessie's Ekans and
James' Koffing evolve into Arbok and Weezing respectively. The
construction is shut down after it is revealed that the construction
site is interfering by the Digletts.