First Sorry if someone has posted this but I been looking for information today after seeing what I seen, I was playing my single player game at around 63-64 hight level and monster mod on normal, and these giant slime mobs started spawning, yes around hight level 60-65, I dont have any mods that effect mobs, so i was wondering if this is a new featuer or if it was a bug? never seen slimes spawn that high up before.
They spawn in swamps at night so you will see lots of them in a swamp. You will also see them in other biomes since they can travel from the swamp to other places.
Minecraft 1.4.4 Slime Bug