Mycelia of Phytophthora cactorum, a fungus unable to synthesize sterols, were incubated with a series of sterols and several common sterol solubilizing agents. Cholesterol, campesterol, and sitosterol promoted growth, while carpesterol, saringosterol, 20 α-hydroxy cholesterol, coprostanol, and 5α-cholestane inhibited growth. Cholestanol, epicholestanol, and 5α-cholestan-3-one had no significant effect on growth relative to the sterol-free control. All steroids were solubilized in ethanol for studying structure-activity correlations. The added steroids were reisolated from mycelia and their identity confirmed by gas liquid chromatography (GLC) or combined gas liquid chromatography-mass spectrometry (GLC-MS). Tween 80 promoted growth, Triton X-100 inhibited growth, and ethanol had no effect on growth relative to a control.