Field survey and laboratory results in the report demonstrate heavy pollution and ideal conditions for development of tastes and odors in the Yellowstone and Big Horn Rivers. No taste and odor problem materialized during the period of study that could not be overcome by normal water treatment plant practices, but high potential was clearly apparent. A summation of major sources of pollution discharged to the various reaches of the stream is given. Laboratory data from stream and industrial waste studies are summarized. Total organic matter, as established by suspended solids, chemical oxygen demand, and 5-day 20C. B.O.D., demonstrates heavy pollutional discharges to the stream.