SUMMARYRhizidiomyces apophysatusZopf, a water mold with anteriorly uniflagellate zoospores, has been studied at the motile zoospore stage, during the process of encystment, and to the point of the first mitotic division of the cyst nucleus. A spiralled banding of the tinsels was discovered, and made it possible to demonstrate that hairs present on the encysted zoospores are the former tinsels of the flagella. The zoospore has the usual cellular organelles which have been described but differs significantly from other fungal zoospores studied in that the ribosomes, which elsewhere occur in a membrane-limited packet, are here free in the cytoplasm but aggregated around the nucleus. The flagellum is attached to a basal body which is the modified form of the larger of two centrioles which lie next to each nucleus in growing cells. The second centriole lies to one side of the basal body in swimming cells and it is suggested that this centriole may be what Koch called a vestigial blepharoplast in other Phycomycetes. Arising from the centrioles are tubules which run below the surface of the zoospore. Another set of fibrils or tubules which is enclosed in a membrane and which probably forms networks within the spore is also described. Nuclear division inR. apophysatusis intranuclear and mitotic with a spindle.