Spirochetes were observed in the midguts of 35% of 147 motile Ixodes dammini from three locations in Lyme and East Haddam, Connecticut. Positive ticks were removed from eastern chipmunks (Tamias striatus), raccoons (Procyon lotor), white-footed mice (Peromyscus leucopus), and a red squirrel (Tamiasciurus hudsonicus). Spirochetes were isolated in fortified Kelly's medium from nine questing or partially engorged I. dammini adults and nymphs and from the bloods of a raccoon and a white-footed mouse. Connecticut isolates from ticks and mammals were serologically indistinguishable from the original Shelter Island, New York strain when cross-tested by immunofluorescence against their mouse antisera. Sera from eight patients diagnosed as having Lyme disease contained antibodies to spirochetes isolated from ticks and mammals. Our finding of serologically and morphologically indistinguishable spirochetes in a raccoon, white-footed mouse and ticks suggests that closely related serotypes are present in wild mammals commonly parasitized by I. dammini, and further supports the claim that a spirochete is involved in the etiology of Lyme disease.