The fish Cymatogaster aggregata, Embiotoca lateralis and Phanerodon furcatus served as definitive hosts for the trematode Telolecithus pugetensis Lloyd and Guberlet. Eggs that were collected from the terminal part of the uterus of mature worms were found to have undergone several cleavages, but complete development was observed only in some eggs that were eaten by the clam Transennella tantilla which served as the first intermediate host. The miracidium that emerged from the egg was oval and covered with long cilia. Except for germ balls, no other internal structures were seen in the miracidium. In the clam sporocysts were found around the intestine in the vicinity of the gonad. No mother sporocyst generation was identified. Immature sporocysts were most frequent in the fall and winter, while mature sporocysts were most common in the spring and summer. The sporocysts were cylindrical, slightly motile, and contractile. Mature sporocyst infections were often of two size groups--one short and the other long. It appeared plausible to the author that the sporocysts may reproduce by transverse fission. Clams that harbored sporocysts were always sterile. Brevifurcate cercariae which developed in the sporocysts left the clam via the excurrent siphon. The cercaria could not swim, instead it moved in a leech-like manner. If a cercaria touched the soft parts of a potential host, it would attach, penetrate within one and one-half to two hours, and become an encysted metacercaria within 24 hours. The following pelecypods that were found in the same environment as the first intermediate host served experimentally as second intermediate hosts: Clinocardium nuttalli, Schizothaerus nuttalli, Transennella tantilla, Macoma nasuta, and Tellina salmonea. In the laboratory the gastropods Acmaea digitalis and Littorina planaxis, which were not found in the same environment as the other hosts, served as second intermediate hosts, indicating that among molluscs host specificity was primarily ecological. In the field and in the laboratory, the clams Tellina salmonea and Macoma nasuta were the most highly infected with metacercariae indicating that there was a certain degree of physiological specificity. / Graduation date: 1964