This paper presents the application of the potentiometric multisensor system for estimation of water toxicity in terms of the bioassay with three different living test organisms: Daphnia magna, Chlorella vulgaris and Paramecium caudatum. Electrochemical signals from the potentiometric system can be effectively treated with PLS regression to yield the estimates of water toxicity with relative errors around 20% which is acceptable precision taking into account the nature of the reference data.