Two species of Leptosynapta, L. clarki and L. transgressor (Reding,1928), have been found in B.C. waters; the latter species has not been fully established as a distinct species. Various biological aspects of these species have been compared. Each species, when alive, showed a distinct external pigment pattern. The pigments of both species were analysed and believed to be naphthaquinone. 98% of adult L. clarki examined had 12 tentacles (range 9-13) while 65% of L. transgressor examined had 10 tentacles (range 8-14). Ossicles varied according to age and location of specimens and were not reliable characters for distinguishing between the species in adult specimens. Immature L. clarki could be distinguished by their primary ossicles which were lacking in immature L.transgressor. Small specimens of L. transgressor (6-42 mm) tended to have anterior and posterior anchors which were of one type while larger specimens (18-103 mm) had longer posterior anchors resembling those of L. clarki. There was some evidence that ossicles were related to environmental factors such as sediment type and exposure. Oocytes of both species developed as sediment temperatures dropped in the fall and winter, and maximum oocyte diameters were reached during minimum temperatures (Dec.Feb.) Mature oocytes of L. clarki were larger (average 404 ~) than those of L. transgressor (average 272 ~) and there were slight differences in sperm shape and length. Fertilization occurred after the seasonal rise in temperature in the spring, though L. transgressor was a month (April) later in breeding than L. clarki (March). L. clarki broods its young at least until the pentacula stage. Evidence that L. transgressor broods its young is inconclusive; only fertilized eggs and very early embryological stages have been found in the females. The release of juveniles from the adult females of L. clarki has been witnessed for the first time. The release takes place through the five anal pores of the adult. L. clarki occurs from Queen Charlotte Islands to central California while L. transgressor has only been found from southern Vancouver Island to Puget Sound. L. clarki is mainly intertidal and often found in Zostera (Eel Grass) beds. It inhabits a wide range of sediments from sandy silt to gravel. L. transgressor is subtidal only (6-40 meters) and inhabits finer sediments ranging from sandy silt to medium sand. The faunal communities associated with each species are quite distinct. The commensal polychaete, Malmgrenia lunnulata has been found on both species of Leptosynapta and the commensal crab, Pinnixa schmitti has been found in the burrows of L. clarki for the first time. rnese results present sufficient information to establish L. transgressor as a distinct species from L. clarki. / Graduate