Midway Games: Finding Plankton:

Before you can get Plankton, you have to complete the Bikini Bottom, Down Town, Tree Dome, Jelly Fish Fields, and Goo Lagoon levels; and have to get all the other letter tiles at Chum World. Go to the area where the back entrance of the "Big Top" is located. You will see a black speck holding up a letter tile. That little black speck is Plankton. You will have to chase him. The best strategy for this is rolling. When you are directly behind him, jump. If you get him, you will get the letter tile . If you lose, he will yell and jump into a vent . You will then have to go back to where you started and try again. Do not try sneaking up on him from behind -- it does not work. Then, you get to go treasure hunting.