We report the results of several surface geophysical surveys of the central part of the Mariana Basin, western Pacific. Analysis of bathymetric and magnetic data along three distinct spreading segments indicates highly asymmetric spreading processes, the half-spreading rate being two to three times more important on the western side of the basin than on its eastern side. Such an asymmetry is as strong at the centers of segments as at their ends, and does not systematically correlate with an asymmetric shape of the rift valley. A half-graben shape of the rift valley is observed only at segment ends. At segments centers, a symmetric graben shape characterizes the valley. The asymmetric shape of the valley at segment ends probably reflects the localization of the tectonic strain due to changes in lithospheric rheology and thickness. Asymmetric spreading is likely accomplished through a different amount of deformation and magma emplacement on both sides of the spreading axis. It may result from the combined effects of (1) asymmetric rheologic, melting and stress conditions imposed by the influence of the pre-existing volcanic arc and its melt sources on the eastern side of the basin, (2) the northwestward movement of the Philippine Sea Plate while the almost vertical Pacific slab in the Mariana subduction zone resists to this motion, (3) a roll-back component at the Mariana subduction zone.