Eggs and coracidia of 11 spp. of diphyllobothriid cestodes were examined for characteristics of possible taxonomic value. On the basis of egg morphology, spp. could be separated according to the environment of their last intermediate host, i.e., marine or freshwater. Embryonic hook measurements were not found to be valid as taxonomic characters. Hook form was fairly diagnostic for Diphyllobothrium lanceolatum, D. cordatum, D. ditremum, and Diplogonoporus balaenopterae; while in D. ursi, D. alascense, D. osmeri, D. dallieae, D. ? dendriticum, Pyramicocephalus phocarum, and Schistocephalus solidus the hook structures were too variable to be of diagnostic value. Plastin cell counts appear to offer considerable promise as a taxonomic tool. Statistical consideration of these show the mean and range are in close agreement for a given sp. irrespective of host occurrence. Observations were made on the living coracidia of most spp. and consideration given the rate and pattern of swimming, and the response to variation in temperature and degree of salinity. The effects of temperature, pressure, and light on the hatching of eggs are discussed.