Reports a first attempt to provide a computation system that will permit a rapid estimate of the amount of hazard to game fish caused by release of fire retardant chemical into streams. Field measurements necessary for application of the system are (1) the amount of retardant (pounds, nitrogen, calculated as NH3) that enters a stream and (2) the discharge rate and average velocity of the stream, both measured at or near the point of retardant introduction. The result is an estimate of the time, or distance, necessary to allow dilution of the pollutant by shear and turbulent mixing to produce nonlethal conditions. Accuracy expected is about plus or minus 75 percent of the result. Despite a large uncertainty, the estimate is valuable and useful. Causes of this uncertainty are discussed and sample computations are presented using U.S. Geological Survey data for several western Montana streams.