During the breeding season sexually active male mouse lemurs produce a trill call used for advertising. The trill call displays a variable, high pitched, complex frequency and amplitude modulated acoustical structure and is given with a high sound intensity. The different sources of its variation in structure and usage have to be determined to understand its complex acoustical design and function. We investigated the vocal morphology of the trill call of males with known age and weight as well as the behavioral context, in order to evaluate sources of its variation. Cluster and discriminant analyses revealed that the structure of trill calls showed a high degree of individual stereotypy in contrast to a high interindividual variability. A set of three distinct variables (fundamental frequency of call onset, bandwidth of 3rd frequency modulation and end frequency of 7th modulation) was sufficient to identify individual senders with almost 90% probability. The structure of the trill call of adult males was independent of age, weight and context and was stable over at least one breeding season. Its individually distinct call structure and use in relation to social status provide the potential for individual recognition and for assessing the “quality” of a male.