A portable direct calorimeter for aquatic animals is described. The unit has been designed to be operated in the laboratory as well as in the field. All parts needed for construction are readily available and inexpensive (about US $1600). The unit delivered reproducible results from the first experiments with freshwater fish. The sensitivity of the calorimeter is 130 mu V mW(-1) or 1.4 mu A mW(-1), the time-constant being about 20 min.