Most apparatus designed to detect avoidance of pollutants by aquatic organisms require visual observations of test organisms in steep pollutant gradients. AGARS (Aquatic Gradient Avoidance Response System) was developed to eliminate these limitations. This system allows animals to choose between one uncontaminated zone and three increasingly toxic zones in a gradient trough that is monitored for extended time periods by infrared light sources, sensors, and a microprocessor. Data are accumulated hourly and processed by a paper tape reader/calculator/plotter system that records the time test animals remain in each zone and compares behavior before and during test exposures. Initial tests indicate that pinfish (Lagodon rhomboides) tested in the apparatus will avoid chlorine-produced oxidants at concentrations of 0.02–0.04 mg/L.Key words: avoidance responses, behavioral bioassays, automated monitoring systems, chlorine-produced oxidants