SUMMARYThe oomyceteLagenidium giganteum, a facultative parasite of mosquito larvae, requires exogenous sterols for the genesis of zoospores when grown on defined or on usual mycological media. Growth media prepared from oil-rich materials such as soy or hemp seed were very effective inducers for zoospores as were the crude oils obtained therefrom when used in conjunction with normal mycological media. The best individual sterols for zoosporangial growth were sitosterol and campesterol, and less effectively ergosterol and cholesterol. A number of synthetic sterols such as cholesteryl phosphate and cholestan-3 beta-ol were also good inducers; sitosteryl glucoside was also utilized. The sterol requirement and the parasitic mode of existence ofLagenidium giganteumwere compared to those of species ofPythiumandPhytophthora.