An automatic radioactivity monitor for gamma rays was operated continuously for three months in the cooling water discharge canal of Consolidated Edison’s Indian Point Plant No. 1. The reliable performance, and particularly the observed sensitivity, indicated the feasibility of this system for automatically recording the radioactive content of environmental water. A similar system was later installed and is being operated routinely as a final check of the plant’s radioactive waste system. The 5-in.-diam × 4-in.-high NaI(Tl) underwater detector used was calibrated with 75 radioactive waste discharges (measured by other means) as standards. The counting efficiency was thereby determined to be 0.26 ± 0.09 counts/sec/pCi/liter. The computed lower limit of detection, for 95% confidence, 5% false alarm, with a 300-sec integration and 10-count/sec background, was 3 pCi/liter.