It was found that the life cycle of Stephanostomum tenue involves the development of rediae and cercariae in the marine snail, Nassa obsoleta, the utilization of the small fish, Menidia menidia notata, as the second intermediate host, and the development of the adult worm in the intestine of the puffer, Spheroides maculatus. Although the puffer may serve as the experimental definitive host, the striped bass, Roccus lineatus, is probably a natural one.About .4 per cent of the Nassa obsoleta observed were infected with this parasite.The excretory system of the cercaria is represented by the formula 2[3+3+3+3+3+3+3].The arrangement of the reproductive organs, some similarity in the excretory systems, and the fact that fishes serve as hosts to the adult worms suggest an affinity of the Acanthocolpidae to the family Allocreadiidae.