The modification of the dilution method proposed by Wilson was employed for determining the approximate numbers of ammonia-oxidizing organisms in various soils. Most of the studies were confined to the variously treated plats of the Agronomy farm of the Iowa Agricultural Experiment Station. The numbers of nitrite-forming bacteria developing during the course of a nitrification experiment varied with the amount and degree of fineness of the limestone applied to the soil employed. The numbers of ammonia-oxidizing, organisms in the profiles of several Iowa soils decreased rapidly with depth in passing from the A1 to the A2 and to the B horizons. A study of the nitrite-forming flora of the variously treated Agronomy farm plats showed that large increases in numbers nearly always occurred where limestone had been applied. In addition to the limestone applications, a combination treatment of manure and rock phosphate was probably the most favorable to these organisms. Manure or crop residues with limestone also usually brought about large increases in numbers over those found in untreated soils. Manure alone favored an increased nitrifying flora in the plats cropped continuously to corn, but no such favorable effect was apparent in the 5-year rotation series. No distinct differences in numbers were in evidence to show that any rotation studied was superior in favoring greater numbers of ammonia-oxidizing organisms than other rotations. The nitrite-forming bacteria in the soils studied reached their maximum numbers in the spring months. The data secured on the numbers of ammonia-oxidizing organisms in soils seem to be in general agreement with the conclusions of many previous reports on the nitrifying powers of soils.