AbstractA comparison was made of the sensitivity of fish,Daphniaand algae to the toxic effects of copper and acridine. A series of toxicity tests was conducted with these organisms, and the following biological endpoints determined: LC50s for fish, LC50s and effects on reproduction ofDaphniaand 50% inhibition of the growth rate of algae. The 96‐h LC50s for bluegill(Lepomis macrochirus)andDaphnia magnaexposed to copper were 2.2 and 0.13 mg/L, respectively. A chronic exposure to 0.03 mg/L of copper for 14 d significantly decreased reproduction inDaphnia.Exposure to 0.4 and 0.2 mg/L copper inhibited the growth rate ofSelenastrum capricornutumandChlorella vulgaris, respectively, by 50%. The 96‐h LC50s for fathead minnows(Pimephales promelas)andD. magnaexposed to acridine were 2.3 and 3.1 mg/L, respectively. A chronic exposure to 1.25 mg/L acridine for 14 d significantly inhibited reproduction inDaphnia, and an exposure to 0.9 mg/L inhibited the growth rate ofS. capricornutumby 50%. Based on the biological endpoints determined in these tests,Daphniawere more sensitive to copper than were fish or algae. In contrast, the most sensitive biological endpoint in tests with acridine was the inhibition of algal growth. Comparison of these test results indicates that short‐term toxicity tests used for screening toxicants for possible environmental effects should include both plant and animal sp