An instrument has been developed which measures the rate of phototaxis in a suspension of microorganisms by recording the increase in optical density which results when the organisms accumulate in a beam of stimulating light. Intensity and spectral quality of the actinic light, as well as of randomly scattered background illumination, can be controlled. Density changes are measured with a dual chopped beam arrangement using infrared light. Thermostatic control of the temperature of the sample is provided.