When known volumes of wet D. desulphuricans were suspended in standard NaCl solutions, the chloride was not diluted by intracellular water below 3% (w/v) NaCl; at 4 and 6% NaCl exchange occurred but not to the extent observed when the organisms were made permeable with cetyltrimethylammonium chloride (CTAC). With intact organisms Na2SO4 up to 15% (w/v) was not diluted by intracellular water. Dilution of 4 and 6% NaCl was due to plasmolytic dehydration of organisms rather than to osmotic breakdown, since phosphate, cytochrome c3 and 265 m[mu]-absorbing material did not escape from the organisms, though they did with CTAC; plasmolysis was not visible microscopically. Hence these sulfate-reducing bacteria are freely permeable neither to chloride nor sulfate. Data on bacterial dimensions and fluid contents derived from this work are recorded; they compare satisfactorily with those obtained by optical measurements.