1. The ultrastructure of frozen etched Nitrobacter winogradskyi cells was examined and compared with thin sections of chemically fixed cells. 2. Frozen etched cells showed two equally thick wall layers interconnected by short strands. This connection was weakened by a chemical prefixation treatment explaining the appearance of an intermediate electron-transparent layer in thin sections of chemically fixed cells. 3. Freeze etching revealed membrane surfaces carrying particles with an approximate diameter of 100 A. Cytomembranes were also observed and showed a spatial arrangement similar to that described in chemically fixed cells. 4. In frozen etched cells the cytoplasm had an uniform granular appearance. When the cells were chemically prefixed the uniform granularity was disturbed by a partial coagulation of the cytoplasm. The same enclosures were identified in the cytoplasm of frozen etched cells as those reported in chemically fixed cells, i. e. PHB and polyphosphate storage granules.