The ultrastructural organization of the zoospores of Phytophthora palmivora was examined using freeze substitution as a potential means of improving upon the images obtained with zoospores preserved by conventional chemical fixation. Despite die technical difficulties encountered, freeze substitution was effective in preserving the overall shape of the zoospores and membranous structures, including the plasma membrane, peripheral cisternae, and the water-expulsion vacuole. Coated pits, not previously reported on the plasma membrane of zoospores of fungi, were found in P. palmivora. The nucleus and cytoskeleton of zoospores were not as well preserved as with conventional chemical fixation. Unique features of freeze-substituted zoospores were compared with structures observed in previous studies of zoospores.