The mesopelagic fishAstronesthes nigerconcentrates in the uppermost centimeters of the sea when both sun and moon are below the horizon. Probability of positive catches increases towards the times of lower culmination of sun and moon and perhaps is highest when both culminations are about synchronous. Positive catches were made only when the moon was near its first or third quarter, but this might be somewhat influenced by the material which was not evenly distributed in relation to moonphases. It is assumed thatA. nigerneeds moonlight for vertical migration, but avoids the surface during illumination.