T'HE MAIN epidemiological features of endemic cholera have been long known, with some of the original observations dating back nearly a century. The available information has been reviewed extensively by Pollitzer (1), and unless otherwise stated, all references are taken from this monograph. By the end of the 19th century, the areas of endemicity had been clearly defined, the nature of the countryside in which it was found was described, the relationship of the disease to the weather, especially the monsoon, had been noted, and the inability of IVibrio cholerae to survive for long periods in water containing large numbers of competing organisms and the potentialities of the village tanks, or ponds of surface water, in spreading infection had been observed. The fact that the cholera vibrio could withstand a very high pH had been discovered and indeed utilized in isolating the organism in pure culture. However, efforts to provide an explanation for these features have so far been unsuccessful. In this paper we have reexamined and confirmed some of these epidemiological features and offer a theory and supporting data to explain them. The theory is that the tanks of water are, in fact, the main means of spread of the infection; the seasonal fluctuations of the disease and possibly the limitations of the endemic area are the results of fluctuations in the pH of the tank water. Examination of a number of tanks over a period of a year has shown that in sunny weather the pH commonly rises from about 7.07.5 in the morning to as high as 10.0 and occasionally to 10.5 in the afternoon. But when it rains this rise is prevented, and the pH may fall below 7.0. These fluctuations are the result of the activity of the algae in the water which liberate either carbon dioxide or oxygen according to the intensity of the light available. This range of pH's would give an advantage to the vibrio in the water over other intestinal organisms and permit them to survive, and, through natural selection, may have been responsible for producing the alkaline-resisting capacities of the organism.