A serosurvey for evidence of rickettsial infections was conducted in the rural populations of several tropical rain forest areas in Sierra Leone and Ivory Coast. Seropositivity rates were surprisingly high in both countries, with more than 7% of the individuals in some districts having antibodies to spotted fever-group rickettsiae. No significant difference was found in the overall prevalence of diagnostic antibody titers to spotted fever-group rickettsiae in Sierra Leone (5.3%) and Ivory Coast (6.2%). However, there was a significant difference (p less than 0.001) in the prevalence of diagnostic antibody titers to typhus rickettsiae in the two countries. There were no marked geographic differences within either country in overall prevalence of rickettsial infections, but there were possible area differences in specific seropositivity rates to typhus- and spotted fever-group rickettsiae in Sierra Leone. In both countries, age and sex differences were important in determining seropositivity, but there was no indication of an age-sex interaction. In Sierra Leone, 59 of the 80 positive sera (73.8%) were from persons age 15 or above (p less than 0.001), and 50 of the 80 (62.5%) were from males (p = 0.05). In Ivory Coast, 33 of the 37 positive sera (89.2%) were from the greater than or equal to 15-age group, and 28 of the 37 (75.7%) were from males (p less than 0.001 for both age and sex). The identification of specific areas endemic for these rickettsial diseases should facilitate the diagnosis and treatment of patients with rickettsial illnesses in West Africa.