Infection with Schistosoma mansoni was studied in 5 troops of olive baboons (Papio cynocephalus anubis) in Gombe Stream National Park, Tanzania. Three troops were infected with S. mansoni. An aggregated distribution of parasites was observed among hosts. Troop membership was found to be the most significant factor influencing parasite prevalence. Age and reproductive status had no significant effect, but there was a trend for males to acquire higher levels of infection. However, age–prevalence curves showed a high infection in young baboons declining in the older baboons. Behavioural components of exposure – as measured in water-contact pattern – may be related to parasite burden. A ‘peak shift’ between infection in different age-classes in the different troops was observed: troops with higher schistosome prevalences displayed an earlier peak in prevalence of infection. The baboon troop with the most contact with people showed highest prevalence of infection possibly due to longer exposure to the parasite than the other troops and/or higher host density.