During a marine geophysical survey of the Bismarck Sea by the Australian Bureau of Mineral Resources in 1970, magnetic, gravity, and seismic reflection recordings were made along north‐south traverses with a spacing of 30 to 40 km. The magnetic data have been interpreted, first by visual inspection of magnetic and topographic trends and then by two‐dimensional computer modelling along typical profiles. The interpretation indicates that the Bismarck Sea is divided into two main tectonic provinces separated by a boundary which roughly coincides with a line joining Manus Island and the Willaumez Peninsula of New Britain. An area of apparently nonmagnetic basement about 10 km wide coincides with a well defined band of shallow earthquakes which runs east‐west across the centre of the Sea. A major boundary is present at the eastern end of the Sea along the west coast of the Gazelle Peninsula of New Britain; it continues along an offset in the band of earthquakes to New Hanover.