Abstract: A swimming pool alarm system is provided for actuating an alarm in response to the movement of a person in a swimming pool. The system includes a transmitter mounted below the surface of a swimming pool to continuously transmit ultrasonic sound waves through the body of water, and a similarly positioned receiver detects sound waves in the water and generates an electrical signal in response thereto. The electrical signal is continuously monitored, and the movement of a person in the pool alters the received ultrasonic sound waves and the corresponding electrical signal to produce an alarm signal for actuating an alarm indicator.