Oxygen consumption of a freshwater goby, Tridentiger obscurus, is known to be higher in grouped fish than in isolated ones (negative group effect), although it is lower in grouped fish than in isolated ones (group effect) in many fishes. The oxygen consumption of the goby was measured with various distances between individual fish in order to examine whether the distance affects the oxygen consumption of the goby or not. When the goby was made possible to see conspecific fish and/or its mirror image at a distance of 5 or 10 cm, the oxygen consumption showed a sustained enhancement compared with the cases of no conspecific fish or of a distance of 15 or 20 cm. This implies that the negative group effect on oxygen consumption is an important factor for the species specific distribution of individual fish in its habitat.