Zoospore cytology of two lettuce strains (from carrot and plantain) and a cabbage strain of Olpidium brassicae (Woronin) Dang, was compared with that of two isolates of Rhizophlyctis rosea (deBary & Woronin) Fischer. Olpidium brassicae zoospores contain four to six mitochondria grouped around the nucleus: microbodies are closely associated with the nucleus and mitochondria and casually associated with lipid bodies in the cytoplasm; ribosomes are scattered in the cytoplasm; a rhizoplast, 0.65 μm long, extends from kinetosome and non-functional centriole to an area near the nuclear membrane and consists of elongated, very fine fibrils crossed by electron-dense layers (striations) of varying thickness and density; the configuration of the layers of the rhizoplast in the lettuce strains differed from the cabbage strain. Rhizophlyctis rosea zoospores contain many (about 21) mitochondria grouped around the nucleus; elongated microbodies are conspicuous on the posterior and lateral sides of the nucleus and extend into the cytoplasm to lipid bodies; ribosomes are concentrated in an area around the nucleus and are partially enclosed by a net-like and folded arrangement of rough endoplasmic reticulum. Rhizoplasts of R. rosea consist of fine fibrils which extend from the kinetosome and non-functional centriole to the nucleus and are crossed by electron-dense layers; in one isolate the rhizoplast is 0.35 μm long and tapers to a point which is connected by fibrils to a flap of the nucleus in a posteriorly located nuclear pocket; in the other isolate it is 1.35 μm long, and expands into a cone-like segment which abuts onto the flattened, posterior face of the nucleus. The significance of the arrangement of organelles, and in particular the rhizoplast, in chytrid biosystematics is discussed.