We monitored electromyographic activity of flight and respiratory muscles in relation to biosonar vocalization in the bat Pteronotus parnellii (Microchiroptera: Mormoopidae). Signals were recorded from the blank muscles (lateral abdominal wall), rectus abdominis, diaphragm, pectoralis and serratus ventralis. Signals were telemetered from flying bats with a small FM transmitter modified to summate the low frequency myopotentials with audio signals from a crystal-ceramic microphone. Activities of all muscles were correlated with vocalizations. A discrete burst of activity in the flank muscles accompanied each vocalization. Myopotentials in the diaphragm occurred between groups of calls and did not coincide with activity of the blank muscles or with vocalizations. Flight muscles were inactive prior to the initiation of fliqht. In Sight, vocalizations and the abdominal muscle activity that accompanied them coincided with myopotentials of the pectoralis and serratus ventralis muscles. We propose that contractions of the flank muscles provide the primary power for the production of intense biosonar vocalizations. Synchronization of vocalization with contractions of the pectoralis and serratus ventralis cooperate in pressurizing the thoracic and abdominal cavities. The use of pressure normally generated in flight facilitates respiration and allows the production of intense vocalizations with little additional energetic expenditure.