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Gas vacuole formation was induced in a strain of Nostoc muscorum when the culture medium was diluted with water. The induction also occurred when a single ingredient of the culture medium, NaNO3, was withdrawn. Replacing this ingredient with one of a number of salts (NaCl, KCl, KNO3) in equiosmolar concentration prevented the induction, but replacing it with glucose or sucrose did not. Increased light intensity also induced gas vacuole formation. The combination of withdrawing NaNO3 and increasing light intensity gave a larger induction than the sum of each separately. Gas vacuoles disappeared about 3 d after formation. Their disappearance was accelerated by addition of chloramphenicol which led to collapse of the constitutive gas vesicles by rising cell turgor pressure. The induction of gas vesicles was accompanied by a number of other morphological changes representing the differentiation of hormogonia. Analysis of the cell protein extracts by polyacrylamide gel electrophoresis showed that de novo synthesis of the gas vesicle protein occurred during gas vacuole induction. A number of other proteins were also formed while others disappeared.