The toxicity of arsenic III, arsenic V, sodium dimethyl arsenate, and disodium methyl arsenate to stoneflies, snails, amphipods, and trout, and the bioaccumulation of these compounds were studied during a 28-day flow-through test. Daphnia magna were exposed for 21 days in static tests to determine life-cycle effects. All animals were exposed to concentrations of approximately 100 to 1000 micrograms/L (as arsenic) of each of the compounds. Arsenic III, the most toxic compound, caused a significant reduction in the survival of amphipods at 1000 micrograms As/L after seven days. None of the compounds significantly affected the survival of other test species after 28 days or reduced young production in Daphnia after 14 days of exposure. The concentration of accumulated arsenic in stoneflies, snails, and Daphnia was as much as 131, 99, and 219 times greater than the water concentration, whereas amphipods and rainbow trout contained arsenic residues similar to the controls. Residues in stoneflies, snails, and Daphnia exposed to 1000 micrograms As/L were higher than those in animals exposed to 100 micrograms As/L, but appeared to reach a steady state after 14 days. Total arsenic accumulation was greatest in organisms exposed to inorganic arsenic, particularly at 100 micrograms/L.