Abstract: The ear tag comprises a male part having a stem with a head portion at one end, the head portion forming an external shoulder, and a female part with an opening through which the head portion can be passed. The area surrounding the opening forms a shoulder which engages with the shoulder formed by the head portion to lock the head portion in place. A hollow boss is formed with the female part and encloses the opening and head portion when it has been passed through the opening. A collar is located within the boss, this collar having an internal rim which abuts the shoulder of the female part. The collar is deformable to enable said head portion, when engaged through said opening, to pass the internal rim and become located with the collar such as to be retained by the collar within the boss.