The effect of 51 different inorganic salts on the growth of threeAzotobacter strains was studied by means of a diffusion technique in which 10−4 mole of each salt was deposited in the centre of a solid medium in a petri dish previously streaked with anAzotobacter suspension. After 1 or 2 days, sterile zones appeared around the point of deposition of the salt, ranging from 5 to 100 mm in diameter. Besides inhibitory effects on growth, effects on slime formation and pigmentation were noted. High toxicity was found for the cations of Cs, Ba, Co, Ni, Cu, Au, Zn, Hg and Tl, and for KIO3, Na2CrO4, Sb2O3SbOCl, Na2SeO3, Na2TeO3 and Na2B4O7. Chlorides or sulfates of Na, K, Mg, Ca and Sr were well tolerated. The cations of Li, Rb, Be, Cr, Cd, Al, In, Sn, Pb, and UO2(NO3)2, VOSO4, NaVO3, Na2WO4, NaBiO3 and Na2HAsO4 had an intermediate effect. The results are discussed against the background of the limitations inherent to the method used. They are further compared with those of a previous study on the tolerance ofChlorella for inorganic ions (den Dooren de Jong, 1965).