Deep seismic refraction and vertical reflection recordings have been made in the southern McArthur Basin, over the Bauhinia Shelf and Batten Trough between Daly Waters and the H .Y.C. mineral deposit, and over the Wearyan Shelf between Borroloola and Westmoreland. In the Batten Trough, McArthur Group rocks have a velocity of 5.81 km/s; no velocity contrast was detected between them and basement. Over the Bauhinia Shelf, 100--200 m of Cainozoic, Mesozoic, and perhaps Cambrian sediments, overlie ?Roper Group sediments (P-wave velocity, V = 4.6 km/s). Below these are probable Tawallah Group rocks (V = 5.8-5.9 km/s). Magnetotelluric measurements define a resistivity contrast, possibly basement, at 6-9 km depth. On the Wearyan Shelf at Borroloola, 370 m of Roper Group (V = 3.58 km/s) overlies 2.9 km of Tawallah Group (V = 5.55 km/s). At Robinson River, 650 m of Cainozoic, thin McArthur Group, and, perhaps, upper units of Tawallah Group (V = 4.81 km/s) were detected. Tawallah Group rocks (V = 5.44 km/s) crop out northwest of Robinson River and are about 2.8 km thick. Basement velocity is 6.04 km/s. Between Robinson River and Westmoreland, basement is 3.5-2.7 km deep. At Westmoreland the McArthur Basin sequence thins against the Murphy Ridge . A layer 260 m thick (V = 3.50 km/s) lies on top of a 2.4 km thick layer (V = 5.44 km/s), and the basement velocity of 5.99 km/s increases to 6.06 km/s towards Robinson River. At mid-crustal depths velocities are 5.9--6.9 km/s , and in the lower crust, to depths of 43 km in the west and 40 km in the east, 6.8-7.5 km/s. Below this, a velocity gradient is interpreted until upper mantle velocities are reached at 43-53 km depth in the west (V = 7.5-8.4 km/s) and 44 km in the east (V = 7.9 km/s). Generally, the crustal structure of the North Australian Craton is characterised by high lower-crustal velocities, broad velocity gradients, and thick crust, which probably evolved from an Archaean continental crust during Proterozoic tectonism.