Tick-borne borreliosis in west Africa. Reported cases of tick-borne relapsing fever due to the spirochaete Borrelia crocidurae are rare in West Africa, and few epidemiological data are available. To see how common relapsing fever is in Senegal thick blood smears from cases of fever of unknown origin and from randomly selected clinic outpatients from a rural dispensary were examined for Borrelia. The prevalence of Borrelia infections in small mammals was also assessed. Borrelia was seen in smears of 12 (0.9%) of 1340 children. All children who tested positive had complained of acute fever. Prevalence was 0% (0/496), 0.5% (2/417), 1.6% (5/308), and 4.2% (5/119) at ages 0-1, 2-4, 5-9, and 10-14, respectively. 26 other instances of borreliosis were seen in patients from different regions of Senegal. Blood samples from 7 of these patients were inoculated intraperitoneally into white mice; serious infection developed in all mice. Borrelia was seen in thick smears from 65 of 461 wild rodents or insectivores. Six rodents species were infected. From a sample of 93 rodents, 33.3% were infected, as judged by intraperitoneal inoculation of white mice, compared with 14.1% by direct smear examination. The findings suggest that borreliosis has a wide distribution and a high incidence in Senegal. This disease may be a major cause of morbidity in rural areas throughout much of West Africa. 