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SUMMARY: When known volumes of wet Desulphovibrio 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 cetyltrimethyl- ammonium 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 plasmo- lytic dehydration of organisms rather than to osmotic breakdown, since phosphate, cytochrome c 3 and 265 mμ.-absorbing material did not escape from the organisms, though they did with CTAC; plasmolysis was not visible microscopically. Hence these sulphate-reducing bacteria are freely permeable neither to chloride nor sulphate. Data on bacterial dimensions and fluid contents derived from this work are recorded; they compare satisfactorily with those obtained by optical measurements.