Organization inside a colony of social insects is based on complex communication media. At present acoustic communication is found to carry various meanings (alarming, reciprocal recognition, other behavioural effects) together with chemical and tactile ones. The five species studied were Ectatomma permagnum Forel, E. quadridens Fabn, E. ruidum Roger, E. tuberculatum Oliver, Pachycondyla apicalis Latreille. The Ectatomma genus, present with 12 species in the tropical forests of Central and South America, has never been studied previously in relation to acoustic emissions. Stridulations were only heard, in the four species considered in this paper, during artificial disturbance of individuals or of the whole colony; so the role of sound production during normal life is still uncertain. Pachycondyla apicalis, while belonging to Central American forests, is also occasionally present in cocoa and coffee plantations. Ultrasonic signals were acquired using a Bruel & Kjaer 2231 phonometer with a B&K 4135 transducer (frequency response up to 100 kHz). Signals were fed into an amplifier with anti-aliasing low-pass filter to be digitally recorded and analyzed on a Pc-based Digital Signal Processing Workstation. Sampling frequencies up to 200,000 s/s allowed the recording up to 87.5 kHz. The ants were picked up with a pincer and the microphone was kept at a distance of approx. 1 cm. All the recordings made under laboratory conditions revealed the emission of pulse trains with very clear pulses extending in frequency up to 75 kHz. The sounds recorded from the genus Ectatomma appeared homogeneous in their acoustic structure, They were typically emitted in long sequences and were made by pulse-trains consisting of two subunits (disyllabic chirps) characterized by pulses with opposite phase, produced by the alternate movement of the simple plectrum against the pars stridens. In Pachycondyla sounds we found sequences of monosyllabic chirps, made by a single train of pulses. Pictures and measurements on the stridulatory apparata were made with a Scanning Electron Microscope Cambridge S 250 TP.