The seismic strain rates and instantaneous velocity fields within the plane of the Tonga slab are investigated using 450 Harvard centroid moment tensor (CMT) solutions from the period 1977--1992. Down-dip seismic strain rates at the bottom of the slab are around 1¿10-15 s-1. Spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab. Solutions show a northward motion of the 200 km depth contour in the Tonga slab at rates of 8¿2 mm/yr relative to the 670 km depth contour of the slab. Relative to the deep, southern portion of the Tonga slab, down-dip velocities increase from near zero in southern Tonga to 10¿2.5 mm/yr in northern Tonga, consistent with the increased subduction rate from south to north described by the Pacific-Australia pole of rotation. Moreover, patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1¿1025 dyn cm. Along-strike extensional strain rates in the deep zone of northern Tonga are approximately 0.5¿10-15 s-1. This is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate, suggesting that the seismic strain rates within Tonga represent a large fraction of the total strain rate budget within the seismogenic zone. An estimated maximum of 60% of the relative motion between the surface and 670 km is absorbed as strain within the slab, indicating considerable slab thickening and penetration of the slab into the lower mantle. ¿ American Geophysical Union 1995