You may be referring to "Breakfast of Champions" by Kurt Vonnegut. There is a Kilgore Trout story in there about a conversation between two yeast as they ate and ate and choked on their own excrement. It's a good book, but not nearly as good as "Slaughterhouse Five" or "The Sirens of Titan", if you're a Vonnegut fan.
I will now become invisible.
-Wesley