Lots of people say that if you keep taking pictures of grimer in the cave level he will evolve into Muk.That doesnt work.(I wasted all my film there, and he STILL didn't evolve). Here is how you really get Muk. You have to take 1 picture of each of the grimer you see at the start of the level. After that you will see 2 grimers on ledges. Hit them with pester balls and he will evolve after you hit him 5 or 6 times.