We present Global Positioning System (GPS) measurements that constrain the amplitude, pattern, and origin of crustal deformation in the Saint Lawrence valley, Qu¿bec, one of the most seismically active regions of eastern North America. The GPS network shows coherent southeastward motion of 0.6 ¿ 0.2 mm yr-1, relative to North America, and uplift of 2.6 ¿ 0.4 mm yr-1. Network average horizontal strain rates are mostly ESE-WNW shortening at (1.7 ¿ 1.0) ¿ 10-9 yr-1. The shortening rate across the Charlevoix seismic zone is about twice as big as the regional average. These measurements are consistent with both postglacial rebound (PGR) models and the deformation style indicated by earthquake focal mechanisms. Although the GPS data do not discriminate between various models of crustal deformation, they provide important constraints on large earthquake recurrence. Assuming that the GPS strain estimates are representative of seismic moment release, they constrain the maximum magnitude of truncated Gutenberg-Richter recurrence statistics in the Charlevoix seismic zone to MX = 7.8 ¿ 0.6 (one M ≥ 7 earthquake per 400--1300 years). The remarkable agreement between the GPS strain rates, seismic catalogue statistics, and PGR predictions suggests that in Charlevoix, most of the PGR-driven crustal strain may be released by large (M ≥ 7) earthquakes. In the rest of the Saint Lawrence valley, PGR strain rates are significantly larger than seismic strain rates, suggesting either that PGR deformation remains mostly elastic or that large events are more frequent than indicated by small earthquake statistics (i.e., characteristic earthquakes).