The ultrastructure of gametangial development and oospore formation was studied in Phytophthora megasperma var. sojae, a species with the paragynous antheridium. Except for the early gametangial interaction, the developmental stages of oogonial expansion, fertilization tube formation, oosphere and oospore formation were very similar to those reported for amphigynous species. The initial contact differs in the two types of gametangial interaction in that in amphigyny the oogonial hypha contacts and penetrates the antheridial initial from the side, whereas in paragyny the antheridial hypha touches and fuses with the oogonial initial from the side without penetrating it. In both cases the end result seems to be the same: the establishment of a firm adhesion zone between the gametangia. Dictyosome-derived vesicles were active in the formation of the contact zone, during oogonial expansion and wall thickening, and during extension of the fertilization tube. An extensive conversion of cytoplasm occurred during differentiation within the oogonium from a largely 'functional' cytoplasm rich in mitochondria, ribosomes, rough endoplasmic reticulum, and dictyosomes to a largely 'storage' cytoplasm within the oospore consisting of an ooplast and surrounding lipid layer.