Year round field studies of the biology of stream sanitation were initiated by the Biology Section of the Public Health Service''s Environmental Health Center on Lytle Creek in October 1949. This stream, which receives the effluent from the primary sewage treatment plant of Wilmington, Ohio, is particularly favorable for studies of the pollutional effects of oxygen-depleting wastes because it has only one source and type of pollution; it has no permanent tributaries below the source of pollution, and it has all degrees of pollution from a a septic zone through recovery and back to clean water. During the summer, a well-defined oxygenless zone occurs in the stream from the sewage plant outfall to a point 2 miles down stream. A recovery zone extends another 2 miles and clean water conditions are reestablished at a point 4 miles distant from the sewage outfall. As many as 40 different spp. of macro-invertebrates were collected in the upper and lower clean water zones with but 15 spp. being taken in the septic zone. All of the organisms occurring in the latter zone had special adaptations for obtaining O2, thus not being restricted in their distr. by O2 depletion. Organisms requiring high O2 concns. as mayflies, stoneflies and caddis flies were restricted entirely to the clear water zones.