At low pH values much of the surface structure ofHalobacterium cutirubrumis maintained, even in the absence of the high salt concentration normally needed to keep this organism from dissolving. If the pH is then raised in solutions of low ionic strength, the cells dissolve. Sphere formation takes place below pH 3 in the absence of salt, and at pH values of about 4 and about 11 in the presence of 4.5 MNaCl. In the latter, there is at first little leakage of intracellular constituents, and the surface of the spheres retains the woven pattern characteristic of the surface of the untreated cell. Below pH 3, this pattern disappears, and the cells become almost completely or completely permeable. In 1% acetic acid, NaCl and KCl have the same effect on cell morphology, in contrast to their differential effects on intact cells. The morphological changes observed appear primarily due to changes in the cell membrane, and are not caused by osmotic pressure.