Agaltepec is a Mexican island, located in Catemaco Lake (Veracruz), where a wild group of mantled howler monkeys was introduced in 1988. At the moment more than 60 individuals live on the island allowing a good opportunity to undertake studies on this interesting colony. The first authors studied the vocal repertoire of the howler monkeys living in Agaltepec. Several groups of A. palliata mexicana were observed for a total of 220 hours and their naturally occurring vocalisations were digitally recorded on a Sony TCD-D7 DAT recorder with a Sony ECM-707 microphone. Sound recordings were processed and then analysed by Sonic Foundry SoundForge 4.5 (PC), Canary 1.4 (Mac) and Voxys 5 (PC). We proceeded to the qualitative and quantitative description of the vocalisations emitted by males by measuring a large set of acoustic parameters, including pulse number, pulse and inter-pulse duration and spectral properties of the emission at the beginning, in the middle and at the end of the signal. As in previous studies, we found several different vocalisations in the study group repertoire (Baldwin and Baldwin 1976: Folia Primatologica, 26, 81-108) but we focussed our study on the descriptive analysis of Roars, Woofs and lncipient Roars. This preliminary analysis of the Alouatta palliata mexicana vocalisations showed that the pulsed structure of the Woofs and of the lncipient Roars shows remarkable difference in pulse number (6.88 vs 16.50), pulse duration (0.6 s vs 0.24 s), interpulse (0.3 s 0.23 s) and total duration (1.22 s vs 3.02 s). Interesting differences in spectral features of the Roar inhaled and expired units are suggested but should be studied in detail in future analyses.