The study was made upon soil collected at depths of 6, 12 and 24 inches on the bank of Pantana Wash, near Tucson, Arizona. The average annual rainfall for 6 years, including the one in which the study was made, was 13.27 inches. Only those forms which grew aerobically upon ordinary laboratory media were studied. The numbers of bacteria per gm. of fresh soil were as follows: 6 inches, 401,000; 12 inches, 1,898,500; 24 inches, 916,500. The highest bacterial count (in the 12-inch layer) was apparently correlated with the highest water content, the largest amounts of combustible material and of soluble chlorides, carbonates and sulphates and the most nearly neutral reaction (pH 8.6). The least favorable conditions were in the 6-inch layer and were correlated with the lowest bacterial count. Of all colonies for all depths, 52.4% were Actinomycetes, which were actually and proportionally more numerous in the 24-inch layer than at higher levels. Of colonies other than Actinomycetes, approximately 88% were white. The numbers of yellow, orange, red, fluorescent and brown forms occurred in the order named. A morphological study of pure cultures fished from the plates showed 24.1% to be cocci (and chiefly in the 6-inch layer); 20.4% short, non-spore-bearing rods; 13.0% long, non-spore-bearing rods; and 42.6% long, spore-bearers (most abundant in the 12-inch layer). Gram-negative forms predominated (64.8%). There was a total lack of gas in sugar fermentation, and almost a total absence of lactose fermenters. Approximately 35% fermented glucose, and 28% sucrose; 70% digested gelatin and 57% casein; 33% reduced nitrates to nitrites.