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MSTRACT: Endproducts of anaerobic metabolism were examined in Nereis diversicolor, N. virens and N. pelagica. These nereids tend to be exposed to different degrees of hypoxia in their natural habitats. Accordingly, they exhibit different degrees of tolerance to anaerobiosis. The present study reveals: (1) In contrast to all previously examined facultative anaerobe annelids all three nereids produce high amounts of D-lactate. (2) N. diversicolor and N. rirens (to a lesser degree) are able to switch, during longlasting anaerobiosis, to the energetically more convenient degradation of glycogen to succlnate and volatile fatty acids. (3) In N. pelagica, however, production of volatile fatty acids is only of minor importance. (4) In all three nereids, alanlne is accumulated during the beginning of anaerobiosis; but in N. virens alanine production continues during longlasting anaerobiosis, although at a lower rate. Variations in responses to anaerobic conditions are also reflected by the 'energy charge', which decreases within 36 h from 0.88 to 0.66 in N. pelagica, whereas in N. virens and N. diversicolor it decreases w~t h i n 72 h from 0.90 or 0.88, respectively, to 0.70. The metabolic pathway of energy production during functional anaerobiosis, caused by extensive muscular work (electrical stimulation), was examined. All three species accumulate D-lactate in considerable amounts; however, succinate or volatile fatty acids were not synthesized.