Shady, Thank you for responding to my post about my cat Ringo. It helps because I think I can now rule out cat acne. The crust is a lot thinner than cat acne, only about one-eighth inch thick. I think it is best described as a continuous crust, not independent globules like cat acne is. Also, it is equally on the top and bottom of his lips, and I think cat acne primarily appears on the bottom lip but seldomly on the top. No real change in the crust today, the fifth day it's been there, and it looks to me like nothing new has accumulated. Instead it looks like the same stuff is stuck there from the past couple of days. So the crust is still on about one-third of his mouth, starting from both corners. It's stuck on the fur beside his lip and doesn't come off easily. Ringo tolerates me wiping with a warm damp cloth for not very long (that's just his springy personality), and I haven't been able to wipe much off but I'll try again. The reason I haven't rushed off to the vet is that in every other way, he appears and behaves just fine. He's eating well, and full of energy like he always is. I've looked at some web sites for cat health but couldn't find information on this condition. If Shady or anybody else has any further ideas, I'd greatly appreciate your sharing them. Thank you so much. --Peter