We determined the acute toxicity of four wildland fire retardants (Phos-Chek 259-Fx, Phos-Chek MVP-Fx, and Phos-Chek LC-95A-Fx, and Phos-Chek LC-95A-R) to two life stages (swim-up fry and young juveniles) of rainbow trout in standardized hard and soft water. The measure of acute toxicity was expressed as both the 96-hour median lethal concentration (96-h LC50, based on mortality) and 96-h median effective concentration (EC50, based on mortality, plus loss of equilibrium and immobilization), which are statistically derived concentrations expected to kill or kill and severely impair, respectively, 50 percent of the test fish in 96 hours. This data set includes the concentration-response data at 24, 48, 72, and 96 hours of exposure for each chemical and measured concentrations of total ammonia in each treatment at 0 and 96 hours. Also included are supportive water quality data measured in each treatment at 0, 48, and 96 hours of exposure and total length and wet weight of the control fish at the end of each test.