Muk basically look very similar to its previous form, Grimer. The main differences are that Muk is larger than Grimer, and has a wider mouth and smaller eyes. It is also usually shown with its left arm being much larger then the right one, along with a strand connecting its upper and lower lip when its mouth is open. The sludge it is made of is apparently less dense than water as it has been shown to float on it in.
The toxicity of Muk's form has reached the point where a single drop of it can make an entire lake stagnant and rancid. Its footprints are toxic enough for a person to contract influenza if they are touched. Their infamous stench increases in the summertime, due to the toxic substances in its body intensifying. Their ability to exist in the environment without it becoming completely hazardous to other lifeforms may be attributed to their ability to actively control their toxins, as they would likely never want to deliberately pollute a river or spread any harmful diseases.
As a fully evolved Pokémon, Muk is capable of using the moves Hyper Beam and Giga Impact.
In the wild, Muks usually take charge of their family of Grimers and are very protective.
This Pokémon likes warm and moist places. Groups of them accumulate in dirty places, such as garbage dumps or sewers. They sometimes also reside in dirty back alleys of towns. Despite their penchant for thriving in dirty, contaminated places, they do seem to have a limit as to how much pollution they can tolerate, as no Muk lived in the Lake Lucid of fifty years ago until some work was made on purifying it. Wild Muk are normally found only in Kanto.
Muk will eat anything that is already contaminated.