The volume of the whole cell and the fraction of the intact cell bounded by the cytoplasmic membrane (protoplast volume) have been measured by dextran and 14C-sucrose exclusion spaces in Neisseria meningitidis competence variants. Increase in external osmotic pressure causes contraction of the protoplast volume. Increasing osmolality due to NaCl and MgCl2 also causes contraction of the volume of the whole cell, whereas increasing concentrations of sucrose cause little or no change in the whole cell volume. The experiments demonstrate a significant difference between competent (cp+) and incompetent (cp-) cells. The cp+ protoplast have a far higher capacity for swelling during decreasing osmolality, and for shrinkage during increasing osmolality. Comparison of spheroplasts obtained by autolysis as well as by the penicillin technique indicates that the average cp+ spheroplast is larger than the average cp- one. The significance of the difference in structure of cp+ and cp- protoplasts has been discussed.