An archeomagnetic study was carried out on potsherds samples from sites in Ontario with ages ranging from A.D. 90 to A.D. 1640 as determined by 14C dating. Thellier double-heating paleointensity experiments were performed in air on 65 specimens of 52 samples from seven sample sets. Reliable paleointensity estimates were obtained for 49 specimens. Alternating field and thermal demagnetization, temperature dependence of weak-field susceptibility, and hysteresis measurements indicate that magnetite of pseudo-single-domain grain size is the carrier of natural remanent magnetization. The paleointensity results follow a half-cycle sine curve, with a steady decrease from 54.0¿5.9 μT to 37.6¿5.7 μT between A.D. 90 and A.D. 885 and a monotonic increase from 52.0¿6.1 μT to 59.4¿1.7 μT between A.D. 1200 and A.D. 1900. The paleointensities determined yield virtual axial dipole moments (VADMs) of the Earth's magnetic field that agree well with those from other parts of North America, except between A.D. 900 and A.D. 1400, when they are systematically lower. This discrepancy is probably caused by a substantial non-dipole field in southwestern North America from the tenth to the fifteenth century, since secular variation studies using potsherds from Arizona and lake sediments from Minnesota show different inclination variations during that period. ¿ 2000 American Geophysical Union