The use of Hydra attenuata in acute toxicity assessment is a potentially useful tool in (waste)water biomonitoring. The purpose of this study was to compare the sensitivity of H. attenuata with the extensively used Microtox® test on 14 (waste)water samples from the Kraków region (South Poland). To this end, specific morphological changes displayed by the freshwater cnidarian Hydra attenuata (lethal LC₅₀s and sublethal EC₅₀s effects) and bioluminescence of the marine bacteria Vibrio fisheri (Microtox®) were compared. Clearly, the Hydra assay was the more sensitive indicator of toxicity. No relationship was found among Hydra toxicological responses and water levels of As, Cd, Co, Cu, Pb and Zn. However, it appeared that toxicity to Hydra might be due to ammonia levels. Additional studies to better circumscribe the tolerance of H. attenuata to ‘natural' water characteristics are needed.