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Diamond and Pearl: Where to Find Drifloon
Drifloon is #065 in the Sinnoh Pokedex and is one of the Pokemon that many people need to encounter to finish their PokeDex after defeating the Elite Four.

Note: It must be Friday and you must have already beaten Mars at Valley Windworks in order to see a Drifloon.

On a Friday during the daytime, fly to Floroma Town. Now head right from the PokeCenter until you get to Valley Windworks. You should see a Drifloon in front of Valley Windworks. You will not encounter Drifloon by walking through grass, you will only encounter one by finding it in the overworld and pressing A to initiate the battle. Drifloon will always be a level 22 female.