COCORP and petroleum industry seismic reflection profiles in northwestern Montana reveal the structure of the Cordilleran foreland thrust belt. The Front Ranges consist of thick thrust sheets containing Precambrian Belt Supergroup and Paleozoic miogeoclinal shelf rocks above a thin remnant of Paleozoic rocks and gently westward-dipping North American basement. Interpretation of the seismic data and results from a recent petroleum exploration well suggest that 15-22 km of Precambrian Belt Supergroup sedimentary rocks are present in several thrust plates beneath the eastern Purcell anticlinorium. Previous hypotheses of a large mass of Paleozoic miogeoclinal sedimentary rocks or slices of crystalline basement located beneath the eastern Purcell anticlinorium do not appear to be supported by the data. The easternmost occurrence of allochthonous basement is interpreted to be in the western part of the anticlinorium near the Montana-Idaho border. Comparison of the Cordilleran foreland thrust belt in northwestern Montana and southern Canada suggests that a change in the deep structure of the Purcell anticlinorium occurs along strike. The anticlinorium in southern Canada has been interpreted as a hanging-wall anticline that was thrust over the western edge of thick Proterozoic North American basement, whereas in northwestern Montana the anticlinorium appears to consist of a complex series of thrust sheets above highly attenuated North American basement.