Cell proliferation is inhibited in many biological systems by lipid peroxides and related products derived from polyunsaturated fatty acids. Using developmentally synchronized cultures of Lagenidium giganteum (Oomycetes: Lagenidiales), a facultative parasite of mosquito larvae, it has been documented that oxidative lipid metabolism is necessary for the induction and subsequent maturation of its sexual stage, the oospore. Addition of lipoxygenase inhibitors to liquid cultures of the fungus results in the stage-specific disruption of antheridial induction, gametangial fusion, induction of meiosis and spore cell wall formation. Oosporogenesis is inhibited by these compounds at concentrations which have no discernible effect on mycelial viability or asexual reproduction. Cyclooxygenase inhibition had comparable effects using ibuprofen and to a lesser extent with indomethacin. Phenylbutazone and the salicylates affected oosporogenesis only at concentrations which decreased asexual reproduction or mycelial viability. The inhibitory effects of NDGA on oosporogenesis could be reversed using partly purified eicosanoid extracts from growth media.