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Protozoa, and particularly ciliates, are essential in aerobic purification processes of wastewaters and have proved to be very sensitive to environmental changes. The physiological response of the ciliate Tetrahymena pyriformiswas assessed in terms of mortality, growth and grazing capacity after exposure to four toxicants: copper, zinc, cycloheximide and Triton X‐100. In the ranges of concentrations used, mortality, inhibition of growth and inhibition of grazing were observed with all toxicants employed, but in different ways. Copper and zinc showed lower toxicity than observed in other studies with protozoa, though some of the present results are in accordance with those reported by other authors. This supports the importance of the organism tested and the experimental conditions of the bioassays.