Sociodemographic and epidemiological data collected on a rural population of the Ziguinchor region of Senegal showed that a large part of the adult population, 80% of women between 15 and 24 years old and 82% of men between 20 and 40 years old, move each year on seasonal labor migrations to the main cities of Senegal or the Gambia or their proximity. In October 1990, an exhaustive seroprevalence survey of the population aged 20 years or older (3,230 persons tested) showed that 0.8% was HIV-2 and 0.1% HIV-1 seropositive. Interviews of 91 persons (24 seropositive persons and 67 seronegative controls) revealed that seropositivity was associated with a history of blood transfusions, injections, sexually transmitted diseases, and seasonal migration. Our findings suggest that in the rural area under study, beside a few cases of transmission by blood transfusion or injection, HIV-2 and HIV-1 are mainly transmitted first to adult men through sexual contacts with infected women met during their seasonal migration and second to their wives or regular partners once they are back home.