We have inverted fundamental and higher-mode Rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over North America to obtain an image of the upper mantle S velocity structure down to 660 km. Among the well-resolved features of the new model are (1) a high-velocity root beneath the North American craton which extends no deeper than 250 km except near the Archean core of the craton where depths of 350 km are reached, (2) a weak band of low-velocity along the eastern margin of the North American craton, which reaches into the transition zone, (3) a low velocity slab window beneath the western United States down to a depth of 300 km, (4) areas of low uppermost mantle velocity beneath the Cascade volcanoes, the Yellowstone hotspot track, the Colorado plateau, the Sierra Madre Occidental, and the grabens bordering the Jalisco block, and (5) a pronounced band of high velocities in the transition zone, coinciding with the expected location of the subducted trailing fragments of the Farallon plate. We introduce several improvements to the method of partitioned waveform inversion, which was used to compute the new model: rather than to correct for an estimated depth to the Mohorovicic discontinuity, we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion; we also improve the windowing and filtering operator used to select uncontaminated waveforms.¿ 1997 American Geophysical Union