Distribution of opine dehydrogenases and lactate dehydrogenase (LDH) among different body parts of a sipunculid are reported for the first time. Alanopine dehydrogenase (ADH), octopine dehydrogenase (ODH) and strombine dehydrogenase (SDH) were present in the body wall and the introvert of Phascolosoma arcuatum. However, no activity of ADH or SDH was detected in the internal organs of this sipunculid. LDH activities were detected in the three body parts studied. In the body wall and the introvert, the activities of LDH represented only a small fraction of the total pyruvate oxidoreductase activities. In the internal organs, however, the activity of LDH was approximately half that of ODH. The lactate oxidizing:pyruvate reducing ratio of the LDH from the internal organs was greater than those from the body wall and the introvert. The electrophoretic pattern of LDH isozymes from the internal organs was also different from those of the latter body parts. The distribution patterns of pyruvate oxidoreductases in these body parts of P. arcuatum were discussed in relation to the physiology of this sipunculid.