Four isolates of rickettsiae from sick dogs on Cape Cod, Mass., were serologically identical to isolates of Rickettsia rickettsii from human patients with Rocky Mountain spotted fever. The antigenic analysis used the indirect fluorescent-antibody test and antisera prepared in mice to each of the isolates and to reference strains of R. rickettsii and Rickettsia montana. Serological responses of infected dogs were specific for R. rickettsii, although antibodies to R. montana were also detected in the sera of most of the canines.