An animal identification device comprising a transponder in the form of a unique electronically coded bolus which fits inside the animals reticulum. A hand held interrogator allows the operator to identify the animals unique number, remotely. To do this, the operator uses his 'password' as verification to allow access followed by which he presses a button which allows him/her to activate the electronically coded bolus, and obtain the animals number. The transmitter system used in the bolus comprises an antenna, oscillator, timer, modulator/tripler and encoder. All the components of the transponder are encased in a cylindrical tube of hard, clear plastics material.