Rediae and cercariae develop in the marine snail, Nassa obsoleta. A small fish, Menidia menidia notata, is the second intermediate host. The adult develops experimentally in the intestine of the puffer, Spheroides maculatus, and, in nature, in the striped bass (Roccus lineatus) also. About 0.4% of the N. obsoleta collected in the Woods Hole, Mass. region was infected with this parasite. The cercaria is of the ophthalmoxiphidio type with a simple tail. The excretory bladder is weakly Y-shaped. The excretory formula of the cercaria is 2 [3 + 3 + 3 + 3 + 3 + 3 + 3]. The arrangement of the reproductive organs and the similarity of the excretory systems suggest an affinity of this family (Acanthocolpidae) to the Allocreadiidae.