The ultrastructure of zoospores of Olpidium brassicae (Wor.) Dang. is described. The zoospores resemble other uniflagellate zoospores in many respects, but ribosomes occur throughout the zoospore body while mitochondria occur only around the nucleus. The axoneme seems to lack a terminal plate, and the rhizoplast differs markedly from that in other chytrid zoospores. The rhizoplast is a cross-banded structure that tapers from 350 mμ wide where it fits against the anterior end of the centriole and kinetosome to 150 mμ near the nucleus; its length is 1 μ. Some zoospores have two or three nuclei, and the origin of these zoospores is discussed.