The fine structure of Phytomonas davidi grown in an axenic medium was analyzed. The results obtained confirm a previous study and add new information. A large membrane-bounded cavity containing vesicles was observed near the nucleus. A vacuole was seen near the flagellar pocket. Its aspect varied from cell to cell and in some cells it was connected to a system of tubules and vesicles and may represent a contractile vacuole. In cells incubated in the presence of 0.1% Triton X-100 and then fixed in a glutaraldehyde solution containing tannic acid, a better visualization of filamentous structures was obtained than with conventional fixation. Filaments connecting the sub-pellicular microtubules to each other and to the plasma membrane were observed. Thin filaments were seen in the cytoplasm, mainly near the basal body. Imidazole-buffered osmium tetroxide solution was used to visualize lipids. Reaction product was seen in the matrix of the peroxisome-like (glycosome) organelle. A high contrast of the membranes which surround the nucleus and the mitochondrion was observed. Other membranes showed the usual aspect.