Grimer can evolve into muk starting at level 38. Grimer's evolution conditions are the same in SoulSilver as every other game so far.
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For Grimer to evolve, get him to level 38.
Grimer evolves into Muk at level 38.
it evolves into a muk* at level 36. :)
Grimer will evolve to a Muk at Level 38.
Your Aron will evolve at level 32 into Lairon.