The growth effects of adding 16 tetracyclic and pentacyclic triterpenoids (at 10 mg/l) to cultures of Phytophthora cactorum, a fungus unable to epoxidize squalene, have been examined. Tetrahymanol, β-amyrin and α-amyrin promoted growth: lupeol, lanosterol, cycloartenol, betulin, and oleanolic acid had no significant effect on growth relative to a control. Hop-22(29)-ene, lupeone, friedelin, and the eucurbitacins a, b, c, d, and e inhibited growth. The ability of certain pentacyclic triterpenoids to stimulate growth was unexpected since the tetracyclic sterol molecule was thought to be necessary for growth stimulation of pythiaceous fungi. That pentacyclic and various tetracyclic triterpenoids inhibit growth of Phytophthora inddicates these compounds may be naturally occurring fungistatic agents.