Clams of the Genus Sphaerium were collected on an island where Leptospira interrogans serotype pomona, infection was enzootic in deer. Leptospires were isolated from both dry, estivating and aquatic, active clams. The isolates did not resemble parasitic leptospires antigenically, and they did not produce disease in gerbils. Clams excreted leptospires into water. Uninfected clams collected from a different source were exposed to serotype pomona in their aquarium. Serotype pomona was recovered from the water after 2 days. Leptospires isolated from fluid in the mantle cavity, digestive gland, and other tissues of the clams appeared to be antigenically different from L. pomona and from each other, and to resemble serotype biflexa Patoc I, serotype semaranga, and a New England strain of water leptospire. It is possible that the molluscan environment modified the expression of genes directing the structure of surface antigens of serotype pomona, and that this modification was stable.