The life cycle of Phytophthora cactorum, a fungal pathogen unable to epoxidize squalene, is controlled by certain combinations and concentrations of polycyclic isopentenoids normally found in the host plant. Data that lend support to this view are given. The fungus is capable of discriminating between various types and amounts of steroids and triterpenoids in terms of their uptake, metabolism, and effects on growth and reproduction. While certain sterols as well as triterpenoids stimulate growth, onlysterols induce significant oospore production. Steroidal alkaloids and estradiol are fungistatic and inhibit sterol-induced oospore production. The primary difference in the metabolism of sterols and triterpenoids is that the sterols are converted to both esters and glycosides, whereas the triterpenoids are only esterified. The results demonstrate the importance of sterols and their products, compared to other polycyclic isopentenoids, as promoters of growth and reproduction and suggest that the preference for sterols may have its origin in the adaptation to host-parasite interactions.