A strain of Strongyloides stercoralis originally isolated from a south-east Asian patient and maintained by passage in monkeys for four years was introduced into parasite-free laboratory-reared beagles. After the second passage in these dogs, the prepatent period shortened from an average of 12-14 d to 8-9 d. Up to the third generation, immunosuppressive treatment of the dogs with oral corticosteroids caused hyperinfection with dissemination, albeit limited, to extra-intestinal organs. From the fourth generation onward, neither hyperinfection nor extra-intestinal migration could be induced even by higher doses of corticosteroids. These observations suggest that the biological behaviour of populations of S. stercoralis can alter as they pass from host to host. Such changes may account, at least in part, for the diverse clinical manifestations of strongyloidiasis.