A 63-year-old, white, animal caretaker suffered intermittent abdominal pain and diarrhea of 3 weeks duration. Hematological findings included leukocytosis (18,600) with eosinophilia (73%). Fecal examination revealed Strongyloides rhabditiform larvae which on culture yielded a preponderance of filariform larvae and a few free-living adults. Thiabendazole therapy resulted in rapid recovery. Epidemiological investigation yielded no history of previous Strongyloides infection or exposure; his wife and pet dog were not infected, but about one-third of the dogs in the colony under his care were found to be discharging Strongyloides rhabditiform larvae in their feces. Strongyloides infection was successfully transmitted to specific pathogen-free pups using filariform larvae derived both from the human case and from the dog colony. Specimens recovered from one pup infected with filariform larvae of human origin were identified as Strongyloides stercoralis.