Systematic relationships within the sea anemone genus Metridium have been problematical for many years. The genus has been considered to consist of a single, highly variable species, M. senile (L.), which has been divided into several somewhat arbitrary morphs. One, the "clonal" morph, is characteristically small, found in shallow water and reproduces both sexually and asexually. Another, the "solitary" morph, may grow very large, is found to great depths and has been thought to reproduce primarily, if not exclusively, sexually. The existence of a second species, M. exilis, described by Hand (1955), has been questioned (Riemann-Zuerneck, 1975). In the present study, the taxonomic status of the morphs and species of Metridium was examined by starch-get electrophoresis to analyze genic variation at 16 enzyme loci in samples of the two morphs of M. senile and M. exilis collected at 3 locations on the Pacific coast of the USA (Bodega Bay, California; Monterey Bay, California; Puget Sound, Washington) and at 1 location on the Atlantic coast of the USA (Maine). Intrapopulation levels of gene enzyme variation were quite high (P0.95 = 47.9, and//= 15.1), and some genetic differentiation among geographically separated populations of a single morph was seen, particularly between Atlantic and Pacific coast populations of the clonal M. senile. However, genetic differentiation (D) among pooled populations of M. exilis, clonal M. senile and solitary M. senile was much greater (/7)= 1.14). Thus, the genus Metridium contains not one highly variable species but three distinct species, each with a relatively narrower ecological range and a relatively less variable morphology.