Aquatic toxicities of six benzimidazole-based anthelmintics-namely, albendazole, thiabendazole, flubendazole, febantel, fenbendazole, and oxfendazole-were evaluated with a marine bacterium, Vibrio fischeri, and a freshwater invertebrate, Daphnia magna. Delayed and chronic toxicity tests using D. magna also were conducted for benzimidazoles with high acute toxicity. Vibrio fischeri was greater than 10-fold less sensitive to most of the benzimidazoles tested compared to daphnids. For D. magna, the most acutely toxic anthelmintic compound tested was fenbendazole (48-h median effective concentration [EC50s], 16.5 microg/L), followed by flubendazole (48-h EC50, 66.5 micro/L), albendazole (48-h EC50, 67.9 microg/L), febantel (48-h EC50, 216.5 microg/L), thiabendazole (48-h EC50, 843.6 microg/L), and oxfendazole (48-h EC50, 1,168.4 microg/L). The lipophilicity parameter, log Kow, explained the observed acute D. magna toxicity of the individual benzimidazoles (r = -0.91, p < 0.01). Delayed expression of toxicity observed for 21 d after 96-h exposure to fenbendazole and flubendazole was not notable, which might result from the relatively high elimination constants for the chemicals. With chronic exposure to fenbendazole, D. magna survival, reproduction, and growth were significantly impacted at 1.25 to 4.1 microg/L (p < 0.05). Hazard quotients estimated for fenbendazole, albendazole, flubendazole, and febantel were 2,770, 9.7, 4, and 1.2, respectively, suggesting a need for further investigation and a potential for environmental concerns, particularly with fenbendazole.