Natural infection with the cephalobid nematode, Daubaylia potomaca, is reported in Helisoma antrosum pericarinatum (= H. anceps pericarinatum) collected in the vicinity of UMBS on Douglas Lake, Cheboygan, Michigan. The life-cycle of D. potomaca, which appears to be unique, has been determined and reproduced experimentally utilizing Australorbis glabratus and other snails as hosts. The cycle involves escape from the host by gravid females, invasion by the females of new hosts and the reproduction therein of generation after generation of parasites, and the eventual emergence of female worms from dying hosts to the external aquatic environment. The parasite does not lead a saprozoic existence, and no dauer larvae were seen. Except for the newly emerged female, no stage of D. potomaca is ’free-living’, and there is no evidence that transmission can occur except through the agency of these females. However, parasitic females removed at any time from their hosts by dissection are fully capable of infecting new hosts under experimental conditions. Dissection of A. glabratus infected with known numbers of female D. potomaca disclosed that the gravid nematodes deposit about two eggs per day in the tissues, that larvae may be found after 5 days of infection, and that the first ’new’ generation of adult males and females appears within the host 11 or more days after infection. Continued reproduction in the host may give rise to very large nematode populations. A. glabratus exposed to five female D. potomaca usually became infected and all infected snails were killed within 44 days. Death of infected snails was preceded, in nearly every case, by the emergence of female D. potomaca from their hosts. It is suggested that this phenomenon reflects an adaptive response of the worms to changes in the internal milieu of the snail. Attempts to infect certain physid and lymnaeid snails with D. potomaca failed, and varying degrees of success were achieved in attempts to infect several planorbids. Of the latter, the most suitable hosts appeared to be A. glabratus (Puerto Rico) and Helisoma sp. (Michigan); the least suitable host was Helisoma caribaeum (Puerto Rico and St. Croix), in which an intense tissue reaction killed all nematode eggs before they could hatch. Other susceptible planorbids (in declining order of apparent suitability as hosts) were Helisoma trivolvis ? macrostomum (Canada), Bulinus truncatus (Egypt) and Planorbarius corneus (Massachusetts). Some of the problems associated with the possible use of D. potomaca as a means of biological control for A. glabratus are discussed.