Energetic studies on exercising animals are usually limited to oxygen consumption measurements in respirometers followed by invasive tissue sampling and analysis of metabolites. Noninvasive studies of exercising animals like through the use of 31P NMR are typically restricted to "stop and go" measurements. Furthermore, magnetic resonance studies of marine animals are hampered by sea water, a highly electric conductive and dielectric medium, resulting in heavy loading and strong RF loss. In this work, we present a set-up for online determination of muscle bioenergetics in swimming marine fish, Atlantic cod (Gadus morhua), using in vivo 31P NMR spectroscopy, which overcome these limitations. Special hardware and RF coils were developed for this purpose. A birdcage resonator adapted to high loadings was used for signal excitation. An insulated inductive coil (2 cm diameter) was fixed onto the surface of the fish tail and placed opposite a watertight, passively decoupled 9 × 6 cm2 elliptic and curved surface coil for signal recordings. This arrangement led to enhanced penetration of the RF signal and an almost 10-fold increase in S/N ratio compared to the exclusive use of the elliptic surface coil. Monitoring of tail beat allowed to set trigger values resulted in an improved quality of in vivo 31P NMR spectra of swimming fish. We report the first successful NMR experiments recording simultaneously tissue energetic and acid-base parameters on swimming cod depending on tail beat frequency and amplitude to determine critical swimming speeds. © 2008 Wiley Periodicals, Inc.