Abstract: A computer interfaced device for rapidly quantifying levels of toxicity to small aquatic organisms in aqueous environments. The device has a plurality of test chambers containing replicates of a plurality of concentrations of a substance or sample being tested for toxicity. Each chamber also contains a predetermined number of motile aquatic organisms. Light originating on one side of each test chamber is received by a photo-detector on the opposite side. The device's sensitivity is adjusted so that each interruption of a light beam caused by movement of a test organism in each test chamber is recorded for that chamber. Using this device and a toxicity testing protocol both lethality and changes in the frequency of test organism's movement in each concentration of substance or sample is monitored. Toxicity is by convention quantified as the concentration of a substance or sample which causes a predetermined level of lethal or sublethal response usually after a plurality of days.