Abstract: A pool safety alarm system includes a water-activated sonar transmitter adapted to be worn on the body of a non-swimmer for continuously transmitting low frequency audio signals upon immersion of the transmitter. An underwater microphone or hydrophone is located within the pool and is connected to a receiver circuit having a band pass filter connected to a monostable multivibrator for supplying signals to an alarm. Whenever signals within the pass band of the filter are received by the hydrophone, these signals trigger the monostable multivibrator from its stable state to its astable state. The astable state of the multivibrator operates the alarm. When signals within the pass band frequency of the filters are received by the hydrophone from sources other than the water-activated transmitter, such signals typically are momentary or intermittent.