The vocal repertoire of adult Antarctic terns consists of 13 different call-types with typical functions, situation specific occurrence and partly individual specificity. I consider the simple calls as possible sources of other vocal signals in away that the most complex signals are derived in a sequence of ritualizations. Most call-types occur in special situations (context specificity) and therefore have special functions. An example for communication by vocal signals (without any optical signal) is given. The fish call series is a typical distance call, which is uttered by Antarctic terns in territorial and mate contexts. That is why this signal is qualified for investigation of species' isolating mechanisms, i.e. for interspecific comparisons. I found differences in the phonetic structure and in the frequency of fish call series between Sterna vittata and other tern species. The allopatric tern species Sterna vittata and S. paradisaea resemble each other in their behaviour about as closely as two distinct species are capable of doing. The innate acoustic signals especially, which are very effective isolating mechanisms, are quite different. That is why the hypothesis that the Antarctic tern is only a subspecies of (Moynihan 1959) or conspecific with (Wolters 1975-82) S. paradisaea can no longer be supported.