For the evaluation of pollutant toxicity in the aquatic environment, a novel principle of a dynamic image analysis of bacterial motion by using a tumbling-ability-defective mutant of Escherichia coil K-12, OW22, isolated as a laboratory indicator organism, is proposed. The average linear motion speed of intact cells of this mutant was approximately 32μ Em/s and decreased markedly by the presence of model chemical pollutants tested. It was suggested that the mutant should be useful as an indicator organism for the evaluation of pollutant toxicity by the dynamic image analysis method.