Electromyogram (EMG) radio transmitters have proven to be a useful tool to monitor activity levels in free swimming fish. Unfortunately, the availability of the EMG transmitter in only radio mode limited its use to the freshwater environment. Applications in the marine environment are numerous and include monitoring activity levels in both wild and cultured finfish. This study presents preliminary data from trials examining activity levels in free swimming sea bass, Dicentrarchus labrax, L., using an acoustic EMG transmitter. Three adult sea bass were surgically implanted with the newly created prototype EMG transmitters. Signals from the transmitter were calibrated to swimming speed using a Blažka-style chamber. Swimming trials showed a high correlation between EMG signal and swimming velocity (r2?=?0.978) and were described using a sigmoid model. No significant differences (P?0.05) were found among the four swimming trials conducted on the same fish or among the trials of the three different fish, indicating minimum variation from the prototypes tested.