The hypothesis that lithospheric structure inherited from the time of continental assembly profoundly influences subsequent tectonism has been strongly supported by recent studies in the middle and southern Rocky Mountains. Compositional differences between tectonic provinces and continued reactivation of the boundaries between them provide significant and long-lived control on the location and style of subsequent deformation. This study investigates correlations between inherited crustal properties, the flexural rigidity of the crust, and observed patterns of deformation in the northern Rocky Mountains. The study area includes most of Montana, Idaho, eastern Washington, eastern Oregon, and southern portions of British Columbia, Alberta, and Saskatchewan. Flexural rigidities are estimated using the Maximum Entropy Spectral Estimation coherence method and range from a low of 8.1 x 10²⁰ Nm in the western Snake River Plain to a high of 1.38 x 10²⁴ Nm in the Columbia Mountains of British Columbia. Flexural rigidity values show significant correlation with crustal age and the location of extensional features. Correlations between flexural rigidity and deformational style along the fold and thrust belt are suggestive but less certain. Inherited crustal boundaries within the area are delineated by zones of crustal weakness with the significant exception of the Vulcan Low. A finger of relatively low flexural rigidities extending northeastward through the Montana Great Plains corresponds closely to the inferred position of the Great Falls Tectonic Zone and supports the hypothesis that a zone of crustal weakness defines this often reactivated boundary between the Archean Wyoming and Hearne provinces. To the south of the area examined in this study, the flexural rigidity estimates of Lowry and Smith (1995) also show a correlation between periodically reactivated, Proterozoic terrane boundaries and areas of crustal weakness. Correlations between these boundaries and low mantle velocities at 100 km depth suggest that these areas may currently be undergoing reactivation by the injection of hot, young mantle material from the west along these preexisting zones of crustal weakness. When used in concert with other geophysical data sets, flexural rigidity mapping can provide a powerful, predictive tool for the investigation of both inherited structure and deformational processes.