An agar plate method, developed for determining the number of Azotobacter colonies in soil, is described. It was used for analyses of a range of fertility plot soils. Greater numbers of Azotobacter were found in limed unfertilized than in limed fertilized soils. Greater numbers were found in soils treated with nitrate of soda than in those receiving additions of superphosphate or chloride of potash. No colonies were found in soils more acid than pH 6.0. Determination of the amount of N fixed by colonies on the plates indicated that the amount of N fixed per colony was inversely proportional to the logarithm of the number of colonies per plate.