The relation of oxygen consumption to ambient temperature and group size was studied in juvenile American shadAlosa sapidissimaat three group sizes: 5 or 6 (small), 9–13 (medium), and 41–48 (large) fish per 500-L tank. Oxygen consumption rates, water temperatures, and behavior were measured concurrently in the three group-size treatments. Schooling predominated in large groups, whereas aggregating behavior dominated medium and small groups. Fish in small or medium groups consumed oxygen at two to four times the rate of fish in large groups (0.36 or 0.25 versus 0.11 mg O2/[g biomass·h]). This group-size-related respiratory rate reduction could be caused by the effect of calming, type of swimming, swimming speed, or hydrodynamic advantage.