In-plant biomonitors, controlled by dedicated microcomputers, may ultimately provide detection of developing toxicity and hazardous spills in industrial effluents rapidly enough to prevent serious damage to receiving waters. A computer automated system was developed to continuously monitor spontaneous locomotor activity of eight crayfish. Electric potentials generated by muscular movements of untethered crayfish were detected by external electrodes, amplified, digitized, and analyzed by a hybrid computer network. A microcomputer evaluated the number of peaks produced in the waveforms of eight individual C. acuminatus for successive one-hour intervals. The resultant counts were directly related to locomotor activity. The effectiveness of the crayfish biomonitor in detecting simulated spills of cadmium in the laboratory was determined by 7 experiments with 56 crayfish. Results indicated that crayfish responded to 2.5 and 0.5 mg Cd⁺⁺/l with increased activity, which was detected by the crayfish biomonitor two hours after exposure. A simulated spill of 0.1 mgCd⁺⁺/l was detected by the biomonitor within 113 hours. These results support the assumption that the crayfish activity monitor may be used as a potential tool for water quality management.