Distance of locomotion of an individual medaka per unit time was measured as an indicator of locomotor activity of the fish. Distance of locomotion in isolated medaka was not different from that in grouped fish at 1 to 3 days post hatch (before development of the group effect on oxygen consumption), and became longer than that in grouped fish at 10 to 13 days post hatch (after development of the group effect). Group effect on oxygen consumption or reduction in oxygen consumption of grouped fish is considered to be caused partly by increased oxygen consumption of isolated fish owing to increases in locomotor activity and metabolic activity, and partly by decreased oxygen consumption of grouped fish owing to the just opposite reasons.