Abstract: The relationship between lunar phase and catch rates of blue marlin, Makaira nigricans Lacépède, 1802, in a diurnal trolling fishery at Yonaguni Island, southwestern Japan, was investigated. The mean catch per unit effort of blue marlin to lunar day was expressed by a periodic regression and significantly increased around the full moon. The stomach content index also significantly increased around the full moon in small blue marlin (<200 cm lower jaw–fork length), indicating that diurnal feeding activities of blue marlin increased around the full moon, especially for smaller individuals. The diurnal feeding activity is thought to be influenced by the nighttime activities of blue marlin and/or prey movements.