The currently referenced age of 730 ka for the Brunhes-Matuyama (B-M) geomagnetic polarity boundary [Mankinen and Dalrymple, 1979> is based largely on K-Ar and paleomagnetic data from postcollapse rhyolites in the Valles Caldera, New Mexico [Doell and Dalrymple, 1966; Doell et al., 1968>. Dating of sanidine from Valles rhyolites by the 40Ar/39Ar laser fusion technique indicates that samples critical to defining the B-M boundary are up to 90 ka older than previously thought. The new data combined with earlier results yield an age for the B-M boundary of 780¿10 ka. 40Ar/39Ar ages from other samples suggest the Jaramillo normal polarity subchron extended from 1010 to 915 ka. ¿ American Geophysical Union 1992