Abstract. The toxicity of pure phenol, formaldehyde, and industrial wastewater, containing phenol and formaldehyde, from a resin production plant was evaluated using aquatic organisms from different taxonomic groups. Test organisms included mixed bacterial culture, unicellular green algae Scenedesmus quadricauda (Turp.) Breb., crustacea Daphnia pulex de Geer (daphnids), and fish Oncorhynchus mykiss Call, 1990 (rainbow trout).Formaldehyde was found to be more toxic to the mixed bacterial culture (120h ECso = 34.1 mg L-L), algae (24h ECso = 14.7 mg L-I), and crustacea (48h ECso = 5.8 mg L -1) than phenol. Phenol proved to be more toxic to fish (48h LCso = 13.1 mg L -~) than to the mixed bacterial culture (120h EC.so = 510 mg L-I), algae (24h ECso = 403 mg L-I), and crustacea (48h ECso = 25 mg L-l). The toxicity of the industrial wastewater to the mixed bacterial culture, algae, and crustacea was caused mainly by formaldehyde, but for fish the presence of phenol in the wastewater proved to be the significant reason for toxicity. Differences in sensitivity of the selected test organisms were also observed, with fish and crustacea being the most sensitive species.