The asexual generations of an echinostome trematode were found in the hemal sinuses of the marine snails, Littorina saxatilis and Littorina obtusata, in the region of Woods Hole, Massachusetts. The cercariae develop in rediae and after emergence encyst in bivalve mollusks. Mytilus edulis and Mya arenaria were employed as experimental hosts. The metacercariae developed to sexual maturity in the intestine of laboratory-reared herring gulls, Larus argentatus, and the worms are described as a new species, Himasthla littorinae. Successive stages in the life cycle are figured. Stunkard (1960) reviewed previous studies on the genus Himasthla Dietz, 1909, con- sidered criteria for specific determination, and discussed the validity of certain described spe- cies. Echinostoma annulatum (Diesing, 1850) Cobbold, 1860, was transferred to Himasthla, and Himasthla tensa Linton, 1940, was sup- pressed as a synonym of Himasthla elongata (Mehlis, 1831) Dietz, 1909. A not fully ma- ture specimen from Nycticorax nycticorax (L.), tentatively assigned to H. elongata, is very similar to Himasthla secunda (Nicoll, 1906) Dietz, 1909. This observation may suggest that H. secunda is a not fully mature form of H. elongata. Metacercariae from the palps and gills of Mya arenaria L., taken in the region of Boothbay Harbor, Maine, developed in the