Magnetotelluric data from almost 200 sites were acquired by a commercial contractor over the Precambrian Purcell Anticlinorium west of the Rocky Mountains of Canada and the United States. Fifteen east–west profiles cross the anticlinorium between latitudes of 48 and 49.5°N, and provide a grid suitable for a regional three-dimensional study of the electrical structure of predominantly the upper crust. The data show essentially a resistive uppermost crust, varying from 2 to 6 km in thickness, over a strongly conductive widespread electrical "basement." The general electrical strike of this conductive basement is found to be N30°W, which concurs with the surface geological trend of the region. The data from all profiles were inverted one dimensionally, and from two of the profiles two dimensionally, using different algorithms, to test the accuracy of the one-dimensional images. The main features found are (i) the sediments in the Upper and Middle Belt–Purcell strata are, in general, more conducting than those in the Lower Belt strata of the anticlinorium; (ii) the basement conductor appears to be strongest just to the south of the Canada–United States border, with resistivities of around 1 Ω·m or less; (iii) the western part of the region is more conducting than the eastern part, suggesting that the sources of sedimentation on the two sides of the region were different; (iv) the enhanced conductivity observed can be explained by the presence of mineralization (copper, etc.), rather than other geophysical causes; (v) near the western edge of the Rocky Mountain trench the conductivity increases downwards from near the surface, and near the eastern edge it increases downwards from a depth of about 2 km, suggesting the presence of the asymmetric mineralization in it; (vi) a few kilometres west of the Rocky Mountain trench in the resistive terrain there exist two narrow, vertical, and significantly conductive electrical "conduits"; and (vii) the pervasive conductive basement extends farther east and north than the present location of the copper sulphide mines in northwestern Montana.