The ultrastructure of the vegetative hyphae and of asexual and sexual reproductive stages of Achlya bisexualis Coker and Couch is studied and is compared with that of other Oomycetes. Also considered are possible structural mechanisms of cellular growth, septation, zoosporogenesis and sexual reproduction. The vegetative hyphae are characterized by an apical growing zone, a sub-apical mitochondrial zone and a distal multi-organelle zone. The apical growing zone contains numerous vesicles, usually to the exclusion of other organelles and inclusions. The sub-apical zone consists of morphologically similar vesicles, but mitochondria and microbodies are abundant in its central protoplasm. The distal multi-organel1e zone is characterized by a variety of organelles and inclusions: dictyosomes, E.R., mitochondria, ribosomes, microbodies, multi-vesicular bodies, lipid droplets, nuclei and centrioles. The dictyosomes occur in association with E.R. and/or nuclei; centrioles appear near modified regions of nuclear envelopes; ribosomes are most abundant in protoplasm nearest actively growing regions; and lipid droplets accumulate in older areas. Liposomes also become evident as hyphal differentiation occurs, and vacuolation of the protoplasm is common. The walls of the vegetative hyphae are composed of a two phase system in which fibrils are embedded in an amorphous matrix. They also are often characterized by pockets of electron dense granular and vesicular material which form during apposition deposition of wall constituents. The gemmae possess the same variety of organelles and inclusions as observed in vegetative hyphae. However, their protoplasm is more dense and zones of morphological organization are not apparent. The walls are extremely thick. The zoosporangia appear as terminal regions of vegetative hyphae which have become delimited from subtending protoplasm by septa. They each usually develop a single apical papilla through which primary zoospore initials escape. The initials are formed by the cleavage of the protoplasm - a process effected by numerous structural changes. There is the development of parastrasomes; nuclei become pyriform and equidistantly spaced from each other and from the sporangial wall; pairs of centrioles differentiate into basal bodies; basal bodies give rise to short axonemes which extend into expanding axonemal vesicles; and, other organelles and inclusions become oriented about the nuclei - the cleavage vesicles particularly becoming aligned and fusing in planes about each of these aggregated masses of protoplasm to form zoospore initials. Encysted primary zoospores differentiate from the initials and characteristic Saprolegniales biflagellate secondary zoospores develop from the primary zoospore protoplasts. The flagellate apparatus in each zoospore is connected with the outer membrane of the nuclear envelope and it consists of an axoneme, basal body and rootlets. The rootlets are thought to be formed, directly or indirectly, by parastrasomes. The gametangia are morphologically distinct from each other. Antheridia are thin-walled and possess a similar pattern of structural organization to that of vegetative hyphae. Their mitochondria mostly appear as rods with terminal or sub-terminal invaginations; liposomes are seldom present in their protoplasm. The oogonia are globose cells and their protoplasm consists predominantly of liposomes and lipid droplets. The nuclei in both antheridia and oogonia divide meiotically. / Science, Faculty of / Botany, Department of / Graduate