Seismic recording along a 600-km traverse across the southern part of the North Australian craton indicates that lateral velocity variations exist in the middle/lower crust. Taken together with other data from the Craton, there is evidence for midcrustal velocities increasing from east to west and from south to north. These trends could result from a greater proportion of high-velocity granulites being emplaced at higher levels in the crust. Between Tennant Creek and Mount Isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites. Near-surface (0--5 km) P wave velocities on the Tennant Creek Inlier increase from 5.47 km/s at the surface to about 6.2 km/s at 6-km depth and correspond with low-grade metamorphics of the Warramunga Group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains. In the Mount Isa Geosyncline, velocities of 6.03--6.15 km/s correspond to the Leichhart Metamorphics interspersed with granites. In the middle crust, P wave velocities of 6.85 km/s occur at a depth of 26 km near Tennant Creek, whereas such velocities are not evident until depths of about 37 km near Mount Isa. There is therefore a trend for midcrust velocities to increase from east to west. The lower crust is characterized by velocities of 7.3--7.5 km/s, and the upper mantle velocities of 8.16--8.20 km/s are reached at depths of 51--54 km. Exposed granulite facies rocks in central Australia are low in heat-producing elements and are compatible with the low surface heat flow generally measured in Precambrian Australia. These data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust, through hornblende granulite, to garnet granulite in the lower crust.