The social determinants and epidemiology of sexually transmitted disease (STD) were studied in rural communities in Mwanza Region, Tanzania, in the context of the phase specific model of STD transmission. The prevalence of HIV and syphilis was higher in communities close to main roads, and lower in communities living on islands in Lake Victoria, probably reflecting the proportion of high risk individuals in the population. The prevalence of Herpes simplex virus type 2 infection, gonorrhoea, chlamydial infection, and trichomoniasis was similar in all types of community, reflecting the fact that these infections remain in the hyperendemic phase. The transmission of STDs is fuelled by high population mobility and by the presence of high risk individuals in rural as well as roadside communities.