Tuna and billfish are highly migratory species, with their habitat and migratory pathways often crossing the Exclusive Economic Zone of different countries. The extent of population structure, habitats and behavior of these fishes are emerging management and conservation issues requiring the attention of fisheries scientists. This project presents the first data on movement, habitat use and behaviour for blue marlin (Makaira nigricans) in the northwest Pacific Ocean. 10 individuals were tracked in the eastern Taiwan using pop-up satellite archival tags (PSATs). Records up to 360 days in length were obtained, providing information on depth and temperature preferences as well as horizontal movements. Tags were deployed from commercial harpoon boats fishing out of Taitung, Taiwan targeting billfish on the southeastern coast. Linear displacements ranged from 58 to 1,529 km from deployment to pop-up locations. The distributions of times at depth were significantly different between day and night. During daylight hours, the fish were typically below the near-surface layer, often at 10 to >50 m, sometimes remaining below the near-surface layer at depth throughout the daylight hours, but often returning briefly to the surface. At night, the fish spent most of their time at or very close to the surface. This pattern of behavior also meant that the distributions of time at temperature were significantly different between day and night, with the fish occupying warmer strata during darkness. Depths and ambient water temperatures visited ranged from 0 to 423 m and 33.5° to 6.8 °C, respectively. However in all cases, the depth distribution appeared primarily limited by an 8 °C change in water temperature. Diel diving patterns also suggested basking behavior. Blue marlin behavior makes them particularly vulnerable to surface fishing gears.