Mantle-derived alkali basalts and their peralkaline differentiates of the late Cenozoic Anahim volcanic belt occur in a 500-km-long, east-west-trending belt that cuts across the predominantly north-northwest structural grain of the accreted terranes in southern British Columbia. Pb and Sr isotopic ratios of these rocks indicate that heterogeneous, suboceanic depleted mantle is present in their source region beneath Alexander, Stikinia, and Kootenay terranes. This widespread suboceanic mantle source is characterized 206Pb/204Pb = 18.62-19.18, 207Pb/204Pb = 15.52-15.63, 208Pb/204Pb = 38.15-38.68, and 87Sr/86Sr = .7026-.7033. These values are similar to those of the source for northeast Pacific Ocean seamounts; that is, the Anahim source is not as depleted as the MORB-source in the northeast Pacific. No evidence exists for the presence of radiogenic, subcontinental mantle in the zone of melting beneath the Anahim belt. Locally, very radiogenic initial isotopic ratios from Bella Bella and King Island centers on the coast, and the Wells Gray-Clearwater centers on the far eastern end of the Anahim belt, reflect contamination from a radiogenic, Precambrian(?) crustal component derived from Alexander and Kootenay terranes, respectively. The mantle source for Anahim belt volcanic rocks appears isotopically continuous with the source for alkali basalts in the Basin and Range of the western United States and distinct from the subcontinental mantle source proposed for some Snake River Plain and Columbia River Basalt Group lavas.