Toxicity of water, sediment and sewage in the Bug River Basin was assessed in 2001 (two campaigns) as part of an international monitoring program originating at the ECE/UN (Economic Commission for Europe/United Nations) Convention on the protection and use of trans-boundary waters. Using standardized testing procedures, a battery of tests including plants, bacteria, rotifers and crustaceans were exposed to water, and sediment and sewage samples were collected from monitoring points within the basin. Surface waters within the basin were classified as non-toxic, with the possible exception of samples collected from tributary rivers in the upper Bug River basin and in Terespol. Many of the sewage samples appeared toxic for indicator organisms. The reaction of indicator organisms (toxicity effect) to exposure in a determined sample differed greatly. Comparing the results of the ecotoxicological studies of surface waters and the sewage discharged into these waters, one can observe a distinct influence of the poor ecotoxicological quality of the sewage on the deterioration of the recipient body, i.e. the surface waters. Sediment samples indicated a rather high toxicity for a significant portion of samples, yet these effects were very close in the presence of indicator organisms. Ecotoxicological monitoring perfectly complements chemical monitoring and gives us the possibility to evaluate the toxic impact of pollution on the environment.