Analysis of seven strains designated as Rickettsia conorii for reactivity with a panel of 12 monoclonal antibodies to surface-protein epitopes of spotted fever group rickettsiae and by Western immunoblotting with standard serotyping sera revealed remarkable antigenic diversity. Rickettsial strains from France, Morocco, Ethiopia, Kenya, South Africa, India, and the USSR differed from one another in reactivity with at least one and as many as five monoclonal antibodies. Simko and Indian strains were similar to one another and differed substantially from other R. conorii strains. All seven strains reacted with three R. conorii-specific monoclonal antibodies. Western immunoblotting demonstrated a major 120-kD protein and a major 135-kD protein in all strains. The principal differences were the presence of a major undenatured 130-kD protein in all strains except Indian and Simko, which had an analogous protein of 124 kD. Immunodominant antigenically related, heat-denatured protein bands of 170 kD (Malish 7 strain), 175 kD (Manuel strain), and 190 kD (Kenya tick typhus, Indian, and Simko strains) were not detected in the M-1 and Moroccan strains. This antigenic diversity is greater than that previously reported for other spotted fever group rickettsial species, suggesting that R. conorii is an older species than R. rickettsii with a longer period of time for evolutionary divergence.