1. There is a considerable amount of observational evidence to show that in a number of bird species, especially colony-nesting sea birds, a bird's recognition of its mate and the mutual recognition of parent and young may, in certain circumstances, be based upon individual peculiarities of voice. However, with the exception of a preliminary account of the matter in the Guillemot, (Uria aalge), there is no analytical evidence to support these conclusions. 2. The Sandwich Tern (Sterna sandvicensis) provides good opportunity for study of one aspect of this problem, namely the individual distinctiveness of the 'fish-call' of the returning parent bird. Different individuals were recorded and the results analysed by sound spectrograph. 3. The fish-call is shown to be composed of three segments of sound; each segment displaying individual characteristics of duration, fine structure, pitch etc. The overall relations between them, the "patterning" of the call, is also shown to be characteristic. 4. The extent to which these separate individual characteristics are recognised as distinctive by birds themselves has not been investigated but it is clear that if the fish-call is to be used as an effective means of individual recognition in a large colony the patterning of the call (in the sense of the 'gestalt' or the overall relationship of the component parts) must play a major role.