The effects of pollutants on the behavior of aquatic organisms are being investigated for the purpose of developing behavioral tests applicable to routine toxicological assays. Although behavioral responses are known to be valuable in detecting the biological effects of certain types of pollutants, behavior has rarely been used in routine assays because of the difficulties in making observations and in quantifying behavioral change. The authors have reduced these problems appreciably through the use of a closed-circuit television system and the quantification of the behavioral record by computer. The behavioral analysis system used is an on-line interactive system capable of analyzing linear and angular parameters of motion for groups of organisms recorded simultaneously. It may be adapted to study material ranging in size from bacteria to fishes.