Bacillus megaterium protoplasts, freed of cell wall by treatment with lysozyme, behaved as osmometers and they were observed to swell and shrink in response to changes in the osmolality of their suspending medium. However, the extent of these volume changes could not be predicted by means of the commonly used van't Hoff-Boyle equation, except for a limited range of osmolalities. In media of osmolality greater than about 4 osmolal, protoplasts were larger than expected; they were relatively unresponsive osmotically and behaved as if they were impermeable to water or else structurally rigid. In media of osmolality less than about 1.7 osmolal, protoplasts were markedly smaller than expected, partly because the largest cells burst and partly because swollen protoplasts lost solutes such as inorganic phosphate. It appeared also that a fair degree of tautness or tension could develop in the membrane of swollen protoplasts since bursting and solute loss tended to occur after rather than during swelling as long as the swelling process was a slow one. A fairly wide distribution of cell sizes was observed in populations of protoplasts prepared from cells harvested during the exponential phase of growth. Changes in average cell volume involved shifts in size distribution within the population; these changes could occur during as well as after wall removal. Liberated protoplasts were found to be larger and osmotically more responsive than the wall-encased protoplasts of whole cells from which they were prepared. In all, it appeared that liberated protoplasts had more surface membrane per cell. Protoplast swelling was essentially fully reversible when it occurred slowly and when extensive leakage of solutes did not occur. Thus the response of the protoplast membrane to slowly applied stress appeared to be one of elastic yield with development of tension. However, when protoplasts were made to swell quickly, brittle fracture of the membrane also occurred, and bursting was evident when membrane extension was less than maximal. The findings presented indicate clearly that the mechanical properties of the protoplast membrane cannot be ignored in considerations of the osmotic responses of protoplasts.