In the present study 670 individuals of Gonatus onyx (Cephalopoda: Teuthoidea) were observed in Monterey Bay, California from a remotely operated vehicle. The vertical distribution of this species was bimodal, with peaks at 400 and 800 m depth during the day and 300 and 500 m during the night. The bimodal distribution reflects a life stage shift between younger, schooling juveniles living in shallower water and older, solitary adults which live deeper. Ontogenetic changes in behavior associated with this life stage shift are reflected in the physiology of the organisms as well. Both aerobic and anaerobic metabolism, as estimated using mitochondrial and glycolytic enzymes, decline with increasing body mass, suggesting reduced locomotory capacity in deeper-living adults. Oxygen consumption rates were also determined in relation to oxygen partial pressure. Oxygen consumption regulation was similar between juvenile and adult squids. The critical oxygen partial pressures (29 to 30 mmHg) correspond precisely to the oxygen concentrations found at the depth of maximal abundance for day and night populations of juveniles and adults, respectively. Behavioral and physiological changes with ontogeny of G. onyx are believed to result from reduced visual predator/prey interactions in the light-limited deep sea.