Signals are important for species recognition. In this study, I examined the acoustic structure of loud calls (“Tchi-faks”) in two populations of closely related lemur species in Madagascar, the Decken's and crowned sifakas (Propithecus deckenii and P. coronatus). Both populations exhibited a strong individual signature in the acoustic structure of Tchi-faks. Furthermore, Tchi-faks clearly differed in the acoustic structure between the two populations. Tchi-faks of Decken's sifakas at Bemahara were, on average, longer and have more energy in lower frequency ranges than Tchi-faks of crowned sifakas at Antrema. This variation is most likely due to anatomical differences of the vocal tract between the two species. However, loud calls of further populations need to be studied in order to understand whether the documented variation in loud calls represents species-specific signatures. In addition, to understanding whether these loud calls are important for species recognition, playback experiments are required to examine if sifakas themselves discriminate between calls of different species.