An indirect calorimetry unit is presented. The unit is specifically designed for field experiments, i.e. all components are small, light and can be powered by battery. The measurement method is the determination of total oxygen consumption in a closed chamber. Four different chambers are used for different sizes of aquatic animals. The complete unit price without a recorder is about US $1600. For data recording in the field, a simple, battery-powered portable computer can easily be connected through an A/D-RS232 converter unit (about US $250-$600) to the oximeter.