The Bismarck Sea is a small marginal sea situated north of the island of New Britain and its associated trench. The land areas which surround it on three sides are island arc structures which have been active episodically since the Middle Cretaceous but which were most active during the Tertiary. Magnetic and sediment distribution maps of the Sea have been compiled and the results of two-dimensional magnetic and gravity modelling along five north-south traverses are presented. The magnetic map shows that the general trend of the anomalies is east-west. Individual anomalies can be traced for more than 150 km, mainly in the eastern half of the Sea, but in the western half persistent anomalies are difficult to discern except along the coast of mainland New Guinea. The water depth in the Bismarck Sea is about 2000 m and Bouguer gravity values are of the order of + 150 mGal. There is a substantial sediment-free area across the centre of the Sea, and sediment thickness increases towards land, reaching 2 km along the north New Guinea coast. Magnetic models and sediment distribution indicate that in the eastern half of the Sea north-south extension is taking place, thus implying northwards movement of the North Bismarck Plate relative to the South Bismarck Plate. The extension is occurring at about 8 cm yr−1 and is single limbed, accretion taking place mainly to the south of the Bismarck Sea Seismic Zone. Earthquake focal-mechanism solutions from the eastern part of the seismic zone, which extends E-W across the Sea, indicate that the N-S movement is being accommodated along two north-west-trending segments of the zone, about a pole to the south-west of the Sea. This interpretation supports the concept of an extensional origin for marginal seas. In addition the mechanics of the process are shown to be (a) different from those at mid-ocean ridges, (b) probably episodic, and (c) possibly reactivated across established trends, thus effectively erasing magnetic lineaments. The western half of the Sea is more complex but some north-south extension has occurred at the seismic zone.