The acute toxicity potential of ten industrial plants located in Toyama Prefecture (Japan) was appraised with a microplate-based assay developed with the freshwater cnidarian Hydra attenuata. Three measurement end points (LC 50 , EC 50 , and TC or ''threshold concentration'') were determined based on specific morphological changes displayed by Hydra under conditions of progressive intoxication. Four effluents were shown to be lethal toward Hydra while eight induced sublethal toxicity responses, LC 50 s varied from 18.8 to ΟΎ100% v/v, while EC 50 s ranged from 15 to ΟΎ100% v/v. Similarly, lethal and sublethal TCs ranged from 17.7 to ΟΎ100% v/v and from 8.8 to ΟΎ100% v/v, respectively. Statistical analyses performed on all toxicity data for the ten effluents confirmed that the sublethal end points (EC 50 s and sublethal TCs), previously unreported to assess complex wastewaters, proved to be more sensitive than the lethal end points (LC 50 s and lethal TCs). This was also reflected by lethality to sublethality ratios, which ranged from 1 to 2.6 (LC 50 /EC 50 comparisons) and from 1.1 to 5.4 (LC 50 /TC comparisons) within a 96 h exposure period. Similar statistical analyses undertaken on 24, 48, 72, and 96 h toxicity data failed to show any significant time-related differences, thereby suggesting that an exposure time as short as 24 h would not diminish test sensitivity. Since Hydra displayed an apparent increase in sensitivity toward a few effluents with time of exposure, however, we would nevertheless recommend a 96 h time frame for this microtest. A correlation was also observed between conductivity and Hydra responses, highlighting a possible link to the presence of toxic metal ions. Based on our study, this simple and cost-effective microassay appears valuable as a (sub)lethal toxicity screening tool for effluents. Additional studies are planned with chemicals and other environmental matrices to better circumscribe its scope of usefulness.