The aim of this work was the assessment of the toxicity of various industrial effluents by a battery of bioassays in order to investigate their potential impact during co-treatment of industrial and municipal wastewaters in an activated sludge plant. The battery test included three test-species as indicator organisms: the protozoan unicellular organism Tetrahymena thermophilla, the crustacean Daphnia magna and the marine photobacterium Vibrio fischeri. The results obtained from the bioassays showed highly toxic effluents originating from processes such as metal coating, chemical and dyeing plants. Daphnia magna exhibited the highest sensitivity, while Tetrahymena thermophilla appeared as the most tolerant species. The response of each organism varied from hormesis affects up to 100% mortality for samples collected from food industry effluents. A specific relation between the chemical analysis data and the bioassay results was not observed. The Student-Newman-Keuls statistical analysis showed that the mean values of bioluminescence inhibition of Vibrio fischeri and mortality/immobilization of Daphnia magna were overlapping for the cases that the organisms were exposed to metal coating and chemical industrial effluents, while in the case of Tetrahymena thermophilla the overlapping mean values included metal coating, chemical and paper industries. Distinctively low mean values were observed for distilleries and food effluents for the cases of low mean values were observed for wastewaters from paper industries and distilleries.