Abstract: An echo tracker clamps a time window for detecting an echo at the position of an echo produced by a mobile wall receiving an ultrasonic pulse signal at a repetition rate F.sub.r. It includes a depth counter (28) which counts in order to define the beginning of the time window in relation to a pulse of the pulse signal and which when it reaches a given value, supplies an enabling signal to activate authorization means (30) and a position detector (32).Enabling means (30) provide an enabling signal for a given time which defines the detecting time window. During this time, the position detector counts until the echo is detected. The value reached at that instant is used to clamp again the time window and to modify the initial value of the depth indicator.The invention is useful in the medical field for measuring the changes of the position of an interface between two tissues.