1. Dunaliella viridis Teodoresco thrives equally well in solutions of NaCl 1 to 4 mol and pH 6 to 9.
2. The organism is sensitive to calcium and magnesium, especially in acid medium.
3. Calcium and magnesium are antagonistic. In a molar solution of NaCl the antagonistic relation Mg:Ca is 4 to 5. In a 4 molar solution of NaCl the proportion becomes many times as great (20:1).
4. Although the strains used in this investigation did not occur in sea water concentrates, the increase in the antagonistic ratio Mg:Ca in which they can live closely paralleled the changes in this ratio which take place when sea water evaporates.
5. The other organisms which occurred in the cultures each show a specific relation to Ca and Mg.
6. The size of the cells of Dunaliella does not decrease with increasing NaCl content.
