The three-dimensional structure of pelagic fish schools was investigated at sea during the night by means of underwater stereo-photography. Jack mackerel Trachurus japonicus and mackerel Scomber sp. were photographed. At night, compact and polarized schools were main-tained and, on the other hand, some fish were dispersed or aggregated in very small schools. Es-timated body lengths ranged from 13 to 31cm with an average of 20cm for a school of jack mackerel and 14 to 23cm with an average of l6cm for a school of mackerel. Within the school, jack mackerel of similar size tended to swim next to each other. The mean distance to nearest neighbour was 1.43 body lengths for the jack mackerel school and 1.51 body lengths for the mackerel school. In the vertical plane, jack mackerel seem to swim at the same level as their nearest neighbours at one moment and take up diagonal positions to them at another. In the horizontal plane, jack mackerel less frequently have nearest neighbours at the side. The density estimated for the jack mackerel school ranged from 6.6 to 19.5 fish/m³.