The morbidity and immunological response to naturally acquired Schistosoma mansoni infection in a population of wild baboons (n = 28) was investigated. Serum obtained from the baboons was assayed for adult worm (SWAP) and schistosome egg (SEA)-specific immunoglobulin (Ig)G and IgM antibodies. The animals were euthanised, perfused to recover adult schistosome worms and schistosome-related pathology was assessed. Nineteen animals (68%) had high serum levels of SWAP-specific IgG antibodies and 15 (54%) had high levels of SEA-specific IgG antibodies. Nine animals (32%) had high levels of SWAP-specific IgM antibodies and six (21%) had high levels of SEA-specific IgM antibodies. Mild schistosome-related pathology was noted in 18 animals (64%). However, adult schistosome worms were recovered from only three animals (10%). The results indicate a high exposure to schistosomiasis for free-ranging baboons inhabiting an endemic area, as evidenced by the high prevalence of parasite-specific humoral antibody response. However, this high exposure is associated with low worm recovery and mild pathology. In addition, parasite-specific IgM antibodies provided a good indicator of an active schistosome infection.