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SUMMARYThe effects of duration of infection and of temperature and nourishment in cultures on development of free-living generations of Strongyloides stercoralis were studied quantitatively. Rhabditiform larvae, 228–353 μm long, were collected from infected dogs with or without prednisolone treatment using the Baermann apparatus. Cultures were carried out by the filter paper test-tube method under the following conditions: incubation temperature 15–40°C and faecal dilution 1:0–1:16. Rhabditiform larvae developed predominantly to free-living females at incubation temperatures of 15–30°C and low faecal dilutions, but filariform larvae appeared mainly under extreme conditions such as high temperature. Recovery rates of filariform larvae were inversely related to those of females. It was remarkable that high temperature, but not low faecal dilution, affected development of filariform larvae. Although the appearance of free-living males was constant in various environmental conditions, the present study indicated an increase in free-living males with the duration of infection. Thus, it seems that free-living males are already fixed as male in the egg stage, and potential female rhabditiform larvae differentiate into free-living females or filariform larvae depending on environmental factors. There is no marked difference in the development of rhabditiform larvae into filariform larvae in either the immunosuppressed dog or the intact dog.