Identification of ELB agent-infected fleas and rodents within several foci of murine typhus in the United States has prompted a retrospective investigation for this agent among human murine typhus patients. This agent is a recently described rickettsia which is indistinguishable from Rickettsia typhi with currently available serologic reagents. Molecular analysis of the 17-kDa antigen gene and the citrate synthase gene has discriminated this bacterium from other typhus group and spotted fever group rickettsiae. Current sequencing of its 16S ribosomal DNA gene indicates a homology of 98.5% with R. typhi and 99.5% with R. rickettsii. Through a combination of restriction fragment length polymorphism and Southern hybridization analysis of rickettsia-specific PCR products, one of five tested patient blood samples was shown to be infected with ELB while R. typhi infections were confirmed in the remaining samples. This is the first reported observation of a human infection by the ELB agent and underscores the utility of PCR-facilitated diagnosis and discrimination of these closely related rickettsial infections.