ALTHOUGH intertidal organisms which cannot breathe air must be regularly subjected to tidal conditions in which they must respire anaerobically, there are no known cases in which these or other animals reduce environmental conditions to the point at which they must respire anaerobically. Indeed such reduction of microhabitat conditions would not be expected because of the relative inefficiency of known anaerobic energy-generating pathways. This report describes a rhythm of alternating aerobic and anaerobic metabolism in the polychaete Nereis virens. The rhythm is circadian under low intensity constant illumination.