Zoospores of Monoblepharella sp. a representative of the order Monoblepharidales, no members of which had ever been studied with the electron microscope, were fixed with glutaraldehyde, followed by OsO4, and their ultrastructure is described. The kinetosome, to which the flagellum is attached, has the appearance of a centriole in cross section. Radiating laterally from the kinetosome is a striated disc which is anchored in a pair of annular lamellae. Microtubules run forward in the cell from a ring in the striated disc. The nucleus is surrounded by a concentration of ribosomes, the latter being partially surrounded and traversed by membranous elements. The zoospores of this Monoblepharella sp. do not have a nuclear cap of the type found in the Blastocladiales. The nature and phylogenetic distribution of nuclear caps in the aquatic fungi is discussed. A highly organized tubular structure at the posterior end of the zoospores is described and its possible function as a photosensitive structure discussed. At encystment the flagellum is withdrawn and cross sections of the flagellum are shown within cysts.