New bathymetric, focal mechanism, and magnetic data are presented in this study of the evolution of the Bismarck Sea region. The boundary between the Bismarck and Pacific plates is composed of at least four segments: two transform faults, one spreading segment, and one “leaky” transform. Magnetic anomalies indicate that the Manus Basin (part of the Bismarck Sea basin) was formed during the past 3.5 m.y. by asymmetric sea-floor spreading. The spreading direction averaged N60°W, and the total opening rate was 13.2 cm/yr. The best-fitting pole of relative motion between the Pacific and Bismarck plates is lat 18.5°S, long 141° E and 4.0°/m.y.