Chikungunya disease is generally recognized in Africa by serosurveys conducted in rural areas. Epidemics are rarely documented. We report two outbreaks in Senegal: one having occurred near Kaffrine in 1996 during an epidemic of yellow fever (YF), the second in Niakhar in 1997. Both diagnoses were conducted by IgM antibodies captures and confirmed by virus isolations. In Kaffrine, a randomised study was carried out on 447 blood donors whose serum was systematically tested for the most frequently encountered arboviruses in Senegal. The incidence rate was higher for the unrecognized Chikungunya infection (35.3%) than for the notified YF infection (21%). In Niakhar, Chikungunya infection was initially detected through three cases having occurred in health workers. A serosurvey was then conducted to define the area of prevalence. We report the clinical forms of Chikungunya virus infections, the interest in detecting IgM for recent arbovirus infections and the duration of these IgM. The difficulties of diagnosis of Chikungunya infection in malaria endemic areas are stressed. The occurrence of arbovirus infections, Yellow fever and Chikungunya viruses, transmitted by the same vectors (Aedes aegypti) in Kaffrine are also discussed.