The Public Health Service has been engaged for some years in studies of various phenomena concerned with the pollution and natural purification of streams. One general purpose of these studies has been to evaluate the intensity of bacterial pollution to be expected from known populations discharging sewage into streams of known discharge and velocity of flow. With this end in view we have collected detailed bacteriological data from two streams of quite different types, the Ohio and Illinois Rivers. Published observations on the Ohio River covered a period ot three years, while those of the Illinois River were continued for a complete year, so that in each case information was obtained throughout an entire seasonal cycle. (Author)