Laboratory studies were conducted with five early life stages of rainbow trout, Oncorhynchus mykiss, to determine the acute toxicities of five fire‐fighting chemical formulations in standardized soft and hard water. Eyed egg, embryo–larvae, swim‐up fry, and 60‐ and 90‐d posthatch juveniles were exposed to three fire retardants (Fire‐Trol LCG‐R, Fire‐Trol GTS‐R, and Phos‐Chek D75‐F) and two fire‐suppressant foams (Phos‐Chek WD‐881 and Silv‐Ex). Swim‐up fry were generally the most sensitive life stage, whereas the eyed‐egg was the least sensitive. Toxicity of fire‐fighting formulations was greater in hard water than in soft water for all life stages tested with Fire‐Trol GTS‐R and Silv‐Ex and for 90‐d‐old juveniles tested with Fire‐Trol LCG‐R. The fire‐suppressant foams were more toxic than the fire retardants. The 96‐h median lethal concentrations (LC50s) were ranked from the most toxic to the least toxic formulation as follows (ranges are the lowest and highest 96‐h LC50 calculated for each formulation): Phos‐Chek WD‐881 (11–44 mg/L), Silv‐Ex (11–78 mg/L), Phos‐Chek D75‐F (218–>3,600 mg/L), Fire‐Trol GTS‐R (207–>6,000 mg/L), and Fire‐Trol LCG‐R (872–>10,000 mg/L). Toxicity values suggest that accidental entry of fire‐fighting chemicals into aquatic environments could adversely affect fish populations.