The teguments of adult Schistosoma mattheei from cattle and buffalo, S. hippopotami from hippopotamus and S. margrebowiei from lechwe were examined by means of scanning electron microscopy for cells possibly engaged in immunological action. Leukocytes were observed on the teguments of the schistosomes from all 4 host species. Although certain of these cells seemed to be fused to the surface membrane of the worms, they did not display pseudopodia. The tegument of certain schistosomes from buffalo exhibited cells the size of platelets with dendritic structures connected to the tegument of the parasite. The results seem to indicate that, as with laboratory hosts, naturally infected domestic and wild hosts are unable to mount an effective cellular response against the tegument of live adult shistosomes. The possible role of platelets in immunology against schistosomes is mentioned.