The influence of cholesterol. (E)-17(20)-dehydrocholesterol, sitosterol, (Z)-17(20)-dehydrocholesterol and 20-isocholesterol on growth and sexual morphogenesis in Phytophthora cactorum has been examined. Optimal growth-response and production of oospores occurred with the 3 former sterols, which posseesed "right-handed" side chains (C22 trans-oriented to C13). Abnormal hyphae and aborted oospores were evident in mycelia cultured with sterols having side chains with "left-handed" structures, i.e., 20-isocholesterol and (Z)-17(20)-dehydrocholesterol. The induction of the sexual cycle lacked a selectivity for stereochemistry in the side chain. The results are interpreted to imply that fungal recognition of the sterol molecule in the reproductive phase of the life cycle is of 2 types: one involves discrimination of stereochemical features of the sterol side chain (oospore production); in the other, no functional significance can be attributed to conformation or configuration of side chain moieties (sexual structure induction). Growth response to dietary sterol seeems to fall into the former category.