We report on selected findings of a qualitative social network study investigating STI/HIV-related risk among migrant fishermen based at one of Indonesia's major fishing ports in Bali. Their activities between fishing trips include drinking parties, watching pornographic videos, and visiting brothels, while condom use is rare. While on board, they plan and anticipate these activities and many insert penile implants. These fishermen run a high personal risk of contracting STI/HIV, and, with their circular migration patterns among Indonesian and foreign ports such as Thailand and South Africa, and with visits back to their rural hometowns and wives or girlfriends in Java, there is a serious risk of disease transmission to the general population. This paper argues that the role that social interactions play in HIV/AIDS-related risks should be considered as important as (if not more important than) individual knowledge, attitudes, and practices in the design of effective STI/HIV prevention programs.