Euglena gracilis was exposed for 24 hr to concentrationsof copper, nickel, lead and zinc ranging from 0.1 to 2.0 mg L-1 and to pentachlorophenol ranging from 0.1 to 10.0 mg L-1. Photosynthetic efficiency (PE) was inhibited by increasing concentrations of the heavy metals. Nickel was foundto be the least toxic of the metals to E. gracilis, whilethe other three metals exhibited similar levels of toxicity. Treatment with pentachlorophenol caused unusual responses of the photosynthetic apparatus probably due to the especially high toxicity of this substance. Both cell shape and motility turned out to be insensitive physiological parameters for the toxicity testing of the substances studied.