1. The oxygen consumption of the solitary tunicate Styela plicata was measured in order to estimate routine metabolic maintenance costs of the animal throughout the year. 2. The acclimatized oxygen consumption of Styela is proportional to the 0.7 power of body weight; this value is independent of the acclimatization temperature. 3. Q10 declines with increasing temperature, averaging 3 between 10° and 20° C, and 1.7 between 20° and 30° C. 4. Disproportionately large metabolic costs of routine activity cannot be invoked to explain the apparent lack of reproduction by Styela plicata during the warmest summer months. 5. The northern limit of Styela plicata is in the vicinity of Cape Hatteras, North Carolina. Winter mortality of adults is likely to limit the northern extension of Styela beyond Hatteras, and dislodgement from substrate during cold (growth inhibited) periods is suggested as one cause of winter mortality. 6. At temperatures greater than 10° C, oxygen uptake of Styela is independent of oxygen tension at normoxic conditions. An analysis of the critical oxygen tension as a function of temperature and body size suggests that ciliary activity may decrease the oxygen diffusion distance in the branchial sac at increased temperatures, and that the surface area per unit volume oxygen consumed may increase with increasing body size because of the demands of filter-feeding on the branchial sac.