Cells ofMicrococcus halodenitrificansgrown in media containing more than 0.7 Msodium chloride appeared normal microscopically, but in 0.6 Msalt many cells were swollen and in 0.55 Msalt most cells were swollen or ruptured. The swollen cells were sensitive to osmotic shock. Calcium or magnesium prevented the cells from swelling and extended the lower limit of growth from 0.55 to 0.3 Msalt. Walls of normal cells contained 6 carbohydrates and 16 amino acids. Qualitative chromatographic analyses indicated that cells grown in 0.55 Msalt contained less tyrosine, diaminopimelic acid, and an unknown ninhydrin-positive, cytoplasmic component than cells grown in 1.0 Msalt. Quantitative estimates indicated that diaminopimelic acid/nitrogen ratio in cells decreased gradually from 0.25 to 0.16 as the salt content of the growth medium decreased from 1.0 to 0.55 M, but that it decreased to 0.003 in cells grown in 0.3 Msalt plus calcium. The results suggest that less cell wall material is produced as the salt concentration in the growth medium is decreased and that calcium has a protective effect on the weakened cells or protoplasts.