Mussels occur in high densities in the Wadden Sea. For the management of the area it is important to know what the impact of the mussel population is on the oxygen budget of the area. Oxygen consumption of a musselbed was measured in a large outdoor tank (200 m³) and in the field using a perspex tunnel over a mussel bed. The measurements in the tank gave values which were comparable to previously published values. The field measurements indicated an oxygen consumption (at night) which was considerably higher (3.9 g O₂ m^-2 h^-1) because of oxygen consumption by geochemical processes and benthic organisms associated with the mussel bed.