1. Concentrations of MgCl2 up to 0.01 M have little effect upon the rate or respiration of Bacillus subtilis; at 0.03 M there is an increase in the rate, while in the higher concentrations there is a gradual decrease. 2. There is a well marked antagonism between MgCl2 and NaCl, and a very slight antagonism between MgCl2 and CaCl2.