Triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether) is widely used as an antibacterial agent in various industrial products, such as textile goods, soap, shampoo, liquid toothpaste and cosmetics, and often detected in wastewater effluent. However, there is a paucity of data on the toxicity of triclosan and its effects on aquatic organisms. In this study, the acute toxicity of triclosan to the Microtox bacterium (Vibrio fischeri), a microalga (Selenastrum capricornutum), a crustacean (Ceriodaphnia dubia) and fish (Danio rerio and Oryzias latipes) was examined. As a result, the MicrotoxR bacterium, crustacean and fish had similar sensitivities towards triclosan toxicity (i.e., IC25 from 0.07 to 0.29 mg/L triclosan). In contrast, the microalga was about 30-80-fold (IC25 = 0.0034 mg/L triclosan) more sensitive to triclosan toxicity than the bacterium and fish. Therefore, triclosan is quite highly toxic to aquatic animals, and is particularly highly toxic to the green alga used as a test organism in this study. This result indicates that triclosan exerts a marked influence on algae, which are important organisms being the first-step producers in the ecosystem; therefore, the possible destruction of the balance of the ecosystem is expected if triclosan is discharged into the environment at high levels.