Abstract: There is described a system for interrogating objects equipped with an automatic transponder in which a fixed station transmits an interrogation signal formed by at least two characteristic pulses and the transponder returns a coded response consisting of a plurality of reply pulses. The time interval between two successively transmitted characteristic pulses or pulse trains represents an item of information which determines which set of addresses is to be selected from a number of such sets stored in a memory, these addresses controlling an address input of a parallel-input, series-output multiplexing circuit which supplies a response code wherein the relative positions of the reply change from one code to the next.