Mewtwo in our Pokedex: http://pokemon.supercheats.com/mewtwo
REQUIREMENTS: Get all 16 Badges.
ABILITIES NEEDED: Surf, Flash & Rock Climb
Mewtwo is in Cerulean Cave. Got get there head out of Cerulean city via the Route 24 bridge and hang an immediate left. Then Surf off the grass Southwards to the cave entrance. In the cave head North and then surf up and left as far as you can go. You'll land by two ladders which you want to ignore and continue South along the bottom of the path until you get to another ladder. Nope to that one also. Head left down the stairs and under the bridge and around until you get to a 4th ladder. Take this one up. At the first fork do up, at the second go right and follow that around until you get to the next ladder, then down twice. Ignore the Rock Climbs and continue on the path taking a right at the fork until you get to some water. Surf to the right and go around to another patch of water and Surf on that and take the first right and you'll end up at Mewtwo.
Legendary Pokemon - Mewtwo