Toxicities of four chemical forest fire retardants, Fire-Trol 100 and 931 (ammonium sulfate or polyphosphate with an attapulgite clay thickener) and Phos-Chek 202 A and 259 (diammonium phosphate with a guar gum derivative thickener) were determined by static and flow-through toxicity tests for fry and fingerling coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri); fingerling fathead minnows (Pimephales promelas), bluegills (Lepomis macrochirus), and largemouth bass (Micropterus salmoides); and mature scuds (Gammarus pseudolimnaeus). In static tests, Phos-Chek formulations were more toxic than either of the Fire-Trol formulations to scuds and all life stages of fish. Acute, 96-h LC50's ranged from 40 mg/l for scuds to more than 1,500 mg/l for fingerling fish. In salmon and trout, fry were generally more susceptible than fingerlings to all formulations; yolk-sac fry were more susceptible than swim-up fry. Toxicities of all formulations to salmon and trout were similar at 6 and 11 C. Un-ionized ammonia is believed to be the most toxic component of these fire retardants.