A record section of long-period U.S. and Canadian observations of multibounce SH waves from a Tonga event is used to study lateral variation in the upper mantle. Source to receiver distances range from 81¿ to 131¿ with triplication patterns apparent in S4 for the western North America stations and S5 for the more distant stations. A corresponding synthetic section explaining the timing separation and waveform distortions for the various multiple S phases was derived by forward modeling. Initial trial models consisted of connecting existing pure path models, assuming ATL (old Atlantic) as an analog for the old Pacific, TNA for the western portion of the United States, and SNA for the shield portion of North America. Isovelocity depth contours were constructed to connect these models and corresponding synthetics were computed following a modified WKBJ method. Lateral heterogeneity is found to exist in the upper 350 km of the Pacific Ocean region and can be modeled reasonably well with a high-velocity lid (Vs=4.7 km/s) which thickens with increasing oceanic crustal age at the expense of the underlying low-velocity zone. Velocities in the upper 200 km of the older southwest Pacific are found to be up to 6% faster than those at comparable depths in the younger tectonic region near western North America. The best fitting model indicates that the upper mantle structure of the old Pacific is similar to that of the old Atlantic, although perhaps slightly slower. ¿ American Geophysical Union 1988