Summary Theoretical seismograms of Rayleigh waves based on several models of mantle structure are compared with actual records for various paths. It is found that the model 8099 of Dorman, Ewing and Oliver explains seismograms for Pacific paths but does not agree with records from Indian-Atlantic ocean paths in the period range shorter than about 100s. The velocity of the Airy phase corresponding to the group velocity maximum is about o.Iokm/s lower for the Indian-Atlantic path than for the Pacific. This difference can be accounted for by reducing the shear velocity at the top of the mantle under the Indian and Atlantic oceans by about o-1-o~~ km/s. The difference between the Pacific mantle and the Continental mantle can be explained either by a reduction in shear velocity of the low-velocity layer under the Pacific ocean or by making the low-velocity zone shallower. I. Introduction Recently, a comparison of the upper mantle structure under oceans and continents was made by Dorman, Ewing & Oliver (1960) based upon the group velocity of mantle Rayleigh waves. The existence of Gutenberg's low-velocity layer under continents and oceans was confirmed. However, a significant difference in the mantle structure of the Pacific ocean and continents was found. These investigators proposed a model for the sub-Pacific mantle in which the top of the low-velocity layer is located at a shallower depth than in Lehmann's model for the mantle under continents. Since the shallower low-velocity layer may indicate shallower isotherms under the ocean and may consequently explain the unexpectedly large terrestrial heat flow through the ocean basin, it is important to study Rayleigh waves in more detail to compare the upper mantle structure under different oceans and continents. The upper mantle from the MohoroviEiC discontinuity to the bottom of the lowvelocity layer controls Rayleigh wave dispersion in the range of period from about 30 s to about zoos. Since there is a group velocity maximum in the dispersion curve of Rayleigh waves in the above period range (Ewing &Press 1956), the actual record