In the king penguin, Aptenodytes patagonicus, incubation and brooding duties are undertaken alternately by both partners of a pair. Birds returning from foraging at sea find their mate in the crowded colony using acoustic signals. Acoustic recognition of the mate maintains and strengthens the mate's fidelity and favors synchronization in the different stages of reproduction. In this study it was found that the king penguin vocalizes in response to the mate's playback calls, but not to those of neighbors or unfamiliar conspecific individuals. To study individual features used by the birds for individual recognition of mates, various experimental signals consisting of synthesized modifications of the mate' s call were played back to the incubating bird. Results indicated that birds attend to the FM profile of the call, in particular its initial inflexion. The frequency modulation shape of the syllable can be assimilated to a vocal signature repeated though the different syllables of the call. King penguins pay little attention to the call' s AM envelope or its absolute frequency.