Causes of tastes and odors that have frequently been reported in water supplies taken from the Montana stretch of the Yellowstone River in autumn were investigated in field studies concerned mainly with the river stretch from Laurel to Glendive, Montana. Sampling was conducted at eight locations along this section of the main stem and in the lower reaches of three tributaries. Data are reported for pollution discharges, chemical features, and biological features, including bottom organisms, algae, and plankton. Entrance of organic pollution was chemically detectable but nowhere did it greatly impair domestic water quality at the times of this investigation. A large role in abatement of pollution effects must be ascribed to life processes of benthic and plankton algae, since their influences upon stream chemistry were quite marked.