Even though numerous previous studies have demonstrated the existence of a relationship between mobility and AIDS, the complex mechanisms subjacent to this relationship still remain relatively unknown. The study presented here is based on a survey carried out in 2000 in the Senegal River valley. It specifically examines the link between migration and risky sexual behaviour in the return zone (risk of spread) by using a conceptual framework which takes into consideration (1) various types of mobility, (2) different social contexts, and (3) the non-migrants. The macro-social level is represented here by the choice of two zones in the Senegal River valley that stand in sharp contrast from the point of view of mobility and socio-economic context. Generally, the analyses show that the net effect of the migratory experience is significant in some social contexts and in relation to the social position of migrants in the return area. International migrants avoid the social risk of stigmatization by remaining faithful to their partners, while internal migrants reduce the risk of infection through frequent use of the condom. The inhibitory influence of migration on risky sexual practices in the return zone could explain the favourable situation of Senegal where the AIDS epidemic is moderate and relatively stable compared to most other countries in the region. Information and education programmes should take the social context into account by promoting responsible behaviour among individuals and community level responses aimed at protection based on solidarity.