A study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the Idaho-Wyoming Overthrust Belt. Samples were taken from the Jurassic Twin Creek unit of the most easterly thrust, the Prospect Thrust, for paleomagnetic study. Detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole (latitude/longitude: 83¿N/286¿E) located on the Tertiary segment of the North American apparent polar wander path (APWP). The age of the paloepole roughly coincides with the age of thrusting on the Prospect Thrust as determined by synorogenic deposits. The magnetization was found to reside in magnetite spherules. Previous data from the Absaroka and Darby Thrust sheets that lie to the west of the Prospect Thrust were reevaluated, and they also indicate a synfolding age for the remagnetization. The corresponding paleopole (64¿N/162¿E) lies on the late Mesozoic segment of the North American APWP. For these more interior thrust sheets, the apparent age of the remagnetization is older than that for the exterior Prospect Thrust sheet, but again roughly corresponds with the inferred age of thrusting for the sampled thrusts. We conclude that renormalization in the Idaho-Wyoming Overthrust Belt is temporally related to deformation, and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events. ¿ American Geophysical Union 1990