Two patients with fatal, laboratory-confirmed Rocky Mountain spotted fever (RMSF) were noted early in their course to have a skin lesion characteristic of a rickettsial eschar. Postmortem immunofluorescent and histopathologic studies demonstrated that the lesions were sites of extensive contiguous infection by Rickettsia rickettsii with associated injury to numerous local blood vessels. Rickettsial vasculitis and occlusive luminal thrombosis were associated with dermal and epidermal coagulative necrosis forming the eschars. Both eschars were noted to be the sites of a bite. In one case the arthropod was identified as a tick. The clinical importance of the search for an eschar in a patient suspected of having RMSF is that diagnostic skin biopsy immunofluorescent demonstration of R. rickettsii may be performed prior to the onset of the rash.