That works with moves, that can be learned by level up.
I know. I was sure, that I saw it on several sites as an eggmove. But after I took another look, it seems I was wrong. -.- @Waynolt
As far as I know, I will get the TM somewhere at the end of the game. But I had hoped to use a toxic Grimer throughout the biggest part of the game. Oh well ...
Thanks you all for your help. I was beginning to doubt my sanity...