Abstract 1. 1. Two versions of a modified volumetric respirometer are described. They were found suitable for measuring oxygen uptake by the adult digenean Proctoeces subtenuis at the oxygen tension of atmospheric air. 2. 2. The experimental conditions remain axenic up to 5–11 hr, without the complication of added antibiotics. 3. 3. The oxygen uptake of P. subtenuis is 0·168 μl/mg fresh wt/hr (±0·013 μl), at 15–17°C. 4. 4. Passive diffusion is adequate to satisfy the oxygen requirements of P. subtenuis at external oxygen tensions above 24 mm Hg. 5. 5. The haemoglobin of this species seems to have no necessary respiratory function under normal conditions.