New long-period dispersion data are obtained from the surface waves generated by the Alaska earthquake of March 28, 1964, and recorded at Isabella, Kipapa, and Stuttgart. Digital techniques were used to isolate phases and determine spectrums over the period band 80 to 670 seconds. Available phase velocity data are now accurate enough to permit us to discuss regional variations which can be attributed to heterogeneity of the upper 400 km of the mantle. Average phase velocities are markedly affected by the character of the continental fraction of the path. Shield areas raise the average phase velocity; tectonic and mountainous areas have the opposite effect. The tectonic-shield distinction is as important as the more obvious continental-oceanic distinction. An average mantle structure, designated CIT 12, is determined for the Mongolia-Pasadena composite great-circle path. The major features of this new mantle model are similar to those determined for the New Guinea-Pasadena great-circle path (model CIT 11), namely, a pronounced and deep low-velocity zone and two discontinuities in the upper mantle at depths near 350 km and 700 km. The two models differ in a way that suggests lower average shear velocities under tectonic regions than under shield areas to depths of the order of 400 km.