Egi, SM. Design of an acoustic telemetry system for rebreathers. Undersea Hyperb Med 2009; 36(1):65-71. Despite the abundance of telemetric applications for ecology, behavior and physiology of marine life, few efforts were reported about the use of acoustic telemetry for SCUBA divers. The objective of this study is to design and test an acoustic telemetry system for monitoring breathing gases of a Drager Dolphin semi-closed circuit rebreather as well as the depth of the diver. The system is designed around a PC based surface Unit and a microcontroller based diver carried module that digitizes the Output Of CO(2) and O(2) sensors located in the inhalation side of the canister. One pair of acoustic modems establishes the data link between the microcontroller and the topside PC. The graphical user interface is written in C# and enables the recording of the diving session as well. The system is calibrated in a hyperbaric chamber and tested successfully with four dives in three different environments Using 100% O(2) and Nitrox (47,9% O(2) - 52,1% N(2)) up to 15 m depth and a distance or 40 m between acoustic modems. The telemetry data cannot be used only for recording physiological data but also provides an important operational safety toot to monitor the rebreather user. The future designs will include actuators for controlling the diluent and oxygen flow to closed circuit mix gas rebreathers.