1. The survival of coliform organisms was studied in river water, either raw or sterilised, kept at different temperatures.

2. For determining the coliform count and the differential coli-aerogenes count in sterilised river water, direct plating of the water on agar, with subsequent study of a number of colonies picked at random, was used. For raw river water the rapid method described by Wilson and his colleagues (1935) was used, which obviates the necessity of plating and of colonial examination.

3. When Bact. coli and Bact. aerogenes were held in stored river water, which was protected from agitation, they underwent a gradual decrease in numbers and finally disappeared. At 37°C. they died out rapidly, but survived for a much longer time at temperatures in the neighbourhood of 0°C. They were able to survive longer in sterile water than in raw water.

4. Observations, however, made on water kept at room temperature and subjected to gentle aeration showed that not only did the organisms not die out, but that they actually multiplied, so that their numbers were considerably higher at the end of two months than at the beginning of the experiment.

5. In raw river water coliform bacilli survived longer at room temperature when kept in the dark than in daylight.

6. On the whole, aerogenes1 proved more resistant than Bact. coli to the environmental conditions provided. This was particularly noticeable in samples kept at room temperature (18°C.). In samples of raw water kept at 37°C. aerogenes proved slightly more resistant than coli, while at 0–2°C. the reverse was true.

7. The general conclusion seems to be that, except at very low temperatures, aerogenes is likely to survive longer in raw river water than Bact. coli.

8. This conclusion is clearly of importance in the interpretation of the coli-aerogenes results in water analysis.