SUMMARYOospore morphology and germination were studied using 17 isolates representing three morphological forms (MF) ofPhytophthora palmivorafrom cacao. Oospore production was most abundant on carrot agar. The shapes of gametangia and oospores produced in crosses between different MF isolates were more variable than those produced when the mated isolates were of the same MF. Mating between the MFs produced oospores with varying capacities for germination. Germination of oospores increased with increasing age. Pretreatment for 1 wk at 12 and at 30 C stimulated germination two-fold over oospores pretreated at 21 C. F1progeny from various matings within and between MFs showed differences in colonial morphology, production and morphology of sporangia, and mating type. However, the lengths of the sporangial stalks of F1progeny from germinated oospores derived from either crosses within or between MFs were all similar to the parental types. Two modes of oospore germination were defined; in one mode, which has not been previously reported, oospores germinated within 24 h by producing many germ tubes, each with a germ-sporangium deciduous with a long stalk. In the other mode, more typical ofP. palmivora, one germ tube formed a single, papillate, nondeciduous sporangium.