Abstract: An echo-ranging boundary system is provided for controlling the access of animals to predefined areas. An electronic transponder collar is affixed to an animal for exclusion from a predefined area. An ultrasonic beacon, defining a transmission reference point, periodically transmits a directional ultrasonic interrogation signal for determining the location of animals provided with transponder collars. The transponder collar emits an ultrasonic reply signal, detected by the station after a period of time defined by the propagation delay of the ultrasonic reply signal. The propagation delay of the reply signal is directly proportional to the distance between the transponder and the interrogation signal reference point. A timing threshold is set at the transmission station corresponding to a desired control distance. Reply signals returned before a device time-out are compared to the timing threshold.