The report gives data from a field survey and laboratory results which 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 practices, but high potential was clearly apparent. A summation of major sources of pollution discharged to the various reaches of the stream is given. Copper, lead and chromium are also present in industrial waste discharges. Amounts do not appear significant, but their presence warrants consideration.