SUMMARY:A series of 825 cultures of coli‐aerogenes bacteria isolated at 30° and a series of 735 cultures isolated at 37° from 645 samples of farm water supplies were classified according to the recommendations of the Coliform Sub‐Committee of the Society for Applied Bacteriology (Report, 1949).Klebsiellaconstituted 50% of the cultures isolated at 30°, whereasEscherichia coliI was the dominant type, forming 57%, among the cultures isolated at 37°. It would thus appear that isolation at 30° is as selective forKlebsiellaas isolation at 37° is forEscherichia. Coli‐aerogenes organisms, mainly 37° negative strains ofCitrobacter freundiiI andK. cloacae, were found in waters of high sanitary quality derived from protected springs and wells; but the coli‐aerogenes microflora of polluted water was dominated byE. coliI, which formed 43% of the isolates at 30° and 76% of those at 37°. The results for a series of fortnightly samples from 11 farm water supplies showed a marked seasonal variation in the incidence of different types isolated at 30°;E. coliI formed a higher proportion in summer than in winter, while 37° negative strains ofKlebsiellaandCitrobacterformed a higher proportion in win