Abstract: A phase encoded transponder circuit is disclosed which may be a passive device capable of response to an interrogating signal and is capable of transmitting an individual unique identification code. The transponder is particularly useful for identifying an individual, such as an animal, associated with the transponder. The code is stored in a memory device which is also loaded with several flags to actuate optional states of the system. One set of flags enable a state in which up to three additional words of the message are transmitted by the transponder. These additional words may be fixed internal data or variable data sent to the circuit by an external data module. Another state permits the importation into the transponder of an unlimited stream of external data from a connected device, so that the transponder can be used to transmit a variety of data about the individual associated with the transponder to the interrogation station.