Medical male circumcision has been shown to reduce the risk of heterosexual transmission of HIV infection in men by up to 60% in three randomised controlled trials. However, not much anthropological literature exists to provide a holistic understanding of sexual behaviour among migrating fishermen who have been circumcised. This qualitative study used cultural ecology theory and anthropological methods to develop a more holistic understanding of Luo fishermen's sexual behaviour after circumcision when they migrate (wimbo) to islands in western Kenya. Results from focus-group discussions show that during wimbo there is a deviation from community norms governing sexual expression, influenced by the belief that circumcision provides protection against HIV infection. Through the exchange of sex for fish, circumcised men access new sexual partners in the destination beaches and engage in risky sexual behaviours without any HIV prevention measures. The processes and practices associated with wimbo may therefore help explain why rates of HIV infection are increasing among fisherfolk despite new interventions to combat HIV. These results have relevant implications for HIV-related intervention and policy in sub-Saharan Africa.