Toxicity tests were conducted with selected compounds listed by the United States Environmental Protection Agency (EPA) as priority pollutants. Acute toxicity information was determined for acenaphthene, arsenic trioxide, cadmium chloride, mercury(II) chloride, silver nitrate, chlordane, endosulfan, and heptachlor. Acute tests were conducted using one or more of the following species: fathead minnows (Pimephales promelas), channel catfish (Ictalurus punctatus), rainbow trout (Salmo gairdneri), brown trout (Salmo trutta), brook trout (Salvelinus fontinalis), bluegills (Lepomis macrochirus), snails (Aplexa hynorum), or chironomids (Tanytarsus dissimilis). Acute values from these tests ranged from a silver nitrate 96-hr LC50 of 6.7 microgram/liter for fathead minnows to an arsenic trioxide 48-hr LC50 of 97,000 microgram/liter for chironomids. In addition to acute tests, a fathead minnow embyro-larval exposure was conducted with silver nitrate to estimate chronic toxicity. The estimated maximum acceptable toxicant concentration for silver nitrate, based on fathead minnow survival, lies between 0.37 and 0.65 microgram/liter.