Abstract: A cattle identification tag for attaching to the ear of an animal comprises a male part which passes through the ear and locks into an eyelet in a female part. The female part has a cap whose hardness is greater than that of the eyelet and has an annular open base which encircles the eyelet in the vicinity of and to the rear of a conical bearing surface of the eyelet which receives and locks a re-entrant conical shoulder of a head on the stem of the male part. The angle of taper of the conical shoulder and conical surface is approximately 45.degree.. The stem has a narrowed region which becomes thinner and elongated in case of an authorized attempt to remove the male part.