Detailed airborne magnetic studies conducted over the region of the S. W. Pacific marginal basins extending from the Solomon Islands to New Zealand suggest that three major phases of basin formation and island arc development have occurred in this region. Development of the Tasman Sea took place during the Late Cretaceous-Paleocene. Development of the basins to the east of the Tasman Sea occurred predominantly during the Oligocene as well as during the Upper Miocene to Recent. The South Fuji Basin, consisting of the Kupe and Minerva Abyssal Plains, is marked by the presence of possibly two RRR triple junction spreading centers that were active between the times of anomalies 13 to 7 (36--25.5 m.y.). The Kupe Abyssal Plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center. The western limb appears to have been subducted beneath the present site of the Three Kings Rise. This seafloor spreading phase (calculated half-spreading rate of 35 mm/yr) was coincident with the overthrusting phase of the New Caledonia ultramafic rocks. During that period, active volcanism along the then continuous Solomons-New Hebrides-Fiji-Lau Island arc was taking place. Magnetic anomalies from 1 to 4 (0--8 m.y. B. P.) are seen to extend along a clearly defined lineation pattern over the North Fuji Basin. Analysis of the magnetic anomalies suggests that during the Upper Miocene, at about the time of magnetic anomaly 4 (8 m.y.), the Solomons-Lau Island arc commences breaking up through the development of an active spreading center (calculated half-spreading rate of 35 mm/yr) located between the New Hebrides and Fuji portions of the island arc. Portions of the northeastern sector of the Minerva Abyssal Plain were subducted beneath the North Fuji Basin during this period between 0--8 m.y. B. P. At the time of about anomaly 3 to 2' (4.5--3.5 m.y.) the development of the North Fiji Basin extended into the Lau Basin and Havre Trough, thus separating the Tonga and Lau Ridges. As a result, a distinct series of active spreading centers developed southward along the axis of the Havre-Lau Basin. The Havre Trough is marked by present seismic and submarine volcanic activity. Residual magnetic anomalies 1--2 (0--1.8 m.y. B.P.) form a symmetrical pattern around the axis of the trough extending south from the Lau Basin to the active Taupo-White Island Volcanic Zone of the New Zealand. Phases of the development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of Pacific Plate motion.