The aim of this work is to amplify the knowledge of acoustic communication in ants. Four myrmicinae species belonging to the genus Messor (i.e. M. capitatus, M. minor, M. structor and M. wasmanni) were tested. Some Messor species have already been the object of a preliminary study only on workers (Schillinger and Baroni Urbani, 1985), but in our work the ultrasonic emission of specimens belonging to the castes of queens, males and workers (minor and major) were recorded. Ultrasonic signals were acquired using a Bruel & Kjaer 2231 with a B&K 4135 transducer (frequency response up to 100 kHz) and a bat detector Ultra Sound Advice S-25. Signals were fed into an amplifier with an anti-aliasing low-pass filter to be digitally recorded and analyzed on a Pc-based Digital Signal Processing Workstation. Sampling frequencies up to 20,0000 s/sec allowed recording up to 87.5 kHz. Ants were held with a pincer and the microphone was kept at a distance of approx. 1 cm. The description and measurements of stridulatory apparata were made by means of S.E.M. (Scanning Electron Microscope Cambridge S 250 TP) analysis on the same specimens used for recordings. In all the individuals investigated, a stridulatory organ occurs in the position regarded as typical of Formicidae: the plectrum on the hind margin of the third abdominal tergite and the file of pars stridens on the pretergite of the fourth. The file is made up of very regular parallel cuticular ridges and, in all the individuals examined, extends for almost the whole length of the pretergite itself, stopping at a very short distance from the anterior margin of the pretergite. The pars stridens shows very sharp margins. Posteriorly, some long bristles occur in proximity of the margins. The hind margin of the third abdominal tergite shows, in its very central part, a thickening connected with the scraper. This thickening makes the scraping action of the tergal margin more effective by conferring rigidity upon this region. For each species it is possible to describe a common general pattern of structure and operation of the organ producing sounds and ultrasonic emission which always have values of maximum frequency higher than 41 kHz. Playback tests are in progress in order to clear up the biological role and significance of the acoustic signalling for the survival of the colony of these species.