A global tectonic regionalization, GTR1, has been constructed for use in seismological studies of aspherical heterogeneity. Three oceanic regions are defined by equal increments in the square root of crustal age (25- and 100-m.y. isochrons), and three continental regions are distinguished according to their generalized tectonic behavior during the Phanerozoic (Precambrian shields and platforms, Phanerozoic platforms, Phanerozoic orogenic zones and magmatic belts). The regionalization is presented both as a tectonic map and by its discretization on a 5° by 5° geographic grid. GTR1 is reasonably successful in accounting for the large-scale geographic variations in body-wave, surface-wave, and free-oscillation data, and it should prove useful in the seismological testing of the competing theories of tectospheric development.