Abstract: An ear tag assembly for attachment to an ear of an animal and for housing a transponder. The assembly comprises a body (1) and a pin (2). The body (1) comprises a chamber (11) in which the transponder is housed. A plug (3) closes the chamber (11). The body (1) is formed from hard plastics material and is generally cylindrical with a strengthening ridge (6) parallel to the long axis. The pin (2) comprises a narrow curved backing component (15) and a shaft (16) with a hardened end for penetrating the ear of the animal. The small size of the body (1), its location in the ear and the shape and material of the body (1) result in a durable ear tag assembly.