Apophallus donicus is examined, its life cycle is described, and certain additions are made to its biology. It is thought, but not proven, that eggs are eaten by the first.intermediate host, the local stream snail FluI menicola virens, and the miracidia escape internally. Mother and daughter rediae stages result, the later producing cercariae which are shed into the water. After burrowing just beneath the skin in suckers (Catostomus macrocheilus Girard), rainbow trout (Salmo 9airdneri Richardson), coho salmon (Oncorhynchus kisutch (Walbaum) squawfish (Ptychocheilus oregonensis Richardson), blackside dace (Rhinichthys osculus nubilus (Girard), redside shiners (Richardsonius balteatus balteatus (Richardson),and possibly other fish, the cercariae encyst and the resulting metacercariae cause black-spot disease in the host. Natural definitive hosts were not found, but laboratory infections included gerbils, white rats, golden hamsters, chickens, catfish (Ictalurus punctatus (Rafinesque), cats, and the author. A rainbow trout was found which had 33 immature Apophallus in its small intestine. All were alive and active, but their entire posterior ends, including all the sex organs, were degenerate and poorly formed compared to the anterior halves of their bodies, and there was no evidence that adults with eggs would have resulted. This trematode is very hardy and adaptable in all stages of its life cycle, e.g., cercariae lived and were active up until eight days after being shed, metacercariae remained infective in fish which had been dead and out of water for 60 hours, and adults lived in tap water at room temperature for 72 hours.