When do you catch Zapdos in Pokemon Yellow?

Once you've gotten at least five badges, go to the Pokémon Center on Route 10, and Surf to the south. Disembark at the first place you're able to, and follow the land to the building, the abandoned Power Plant. It's a maze inside, and Zapdos waits at the end of it at Level 50; be sure that you're ready before you take it on.