Domestic and wild animals were examined for natural infection with human schistosomiasis in Northern Gezira, Sudan. Postmortem examination of animals in a number of slaughterhouses revealed no infections in either sheep or goats but Schistosoma mansoni worms were found in two of the 98 cattle examined. Nile rats (Arvicanthus niloticus) were trapped and stray dogs were shot and examined for evidence of schistosome infection. The infection rate in Nile rats was 4.9% (8/164) and in dogs 27.3% (15/55) but whereas viable eggs were found in the excreta and tissues of the rats no eggs could be found in dog faeces. In the laboratory nine puppies were each exposed to 10 000 cercariae but there was no evidence that the infection had taken. It is concluded that A. niloticus, present in large numbers and often found in or near the canals, may be involved in S. mansoni transmission and could act as reservoir hosts, but that dogs are probably infected only in exceptional situations and are unlikely to play a major role in S. mansoni transmission.