Specimens (197) from 122 cores from 23 sites were collected from the 33-km length of the Z-shaped Aiyansh lava flow in the Nass valley of northwestern British Columbia, Canada. The specimen and core remanence directions are very consistent and give a mean of 14.0¿, +72.9¿ (α95=0.8¿; k=1372) after af demagnetization based on site mean directions. A field slump block test, thermomagnetic tests, and other data indicate that the flow has a very stable, uniform primary thermoremanent magnetization residing mainly in the matrix magnetite. The site mean maximum axes of the ellipsoids of anisotropy of magnetic susceptibility are not preferentially aligned along the flow direction of the lava. The flow was extruded in 1650¿40 A.D. based on the flow's remanence declination and extrapolation of observatory records. This refines the Indian legend and tree ring date of about 1770 A.D. and the 14C date of 1700¿130 A.D. The virtual geomagnetic pole for the Aiyansh lava flow is 68.0¿W, 81.7¿N(P0.05:Δm=1.3¿).