Paleomagnetic data were acquired from 58 Miocene volcanic units exposed in five mountain ranges in southwestern Arizona. Unlike results of similar paleomagnetic studies west of the San Andreas fault, no clockwise rotations of the paleomagnetic vectors are observed. In fact, the data indicate a statistically significant counterclockwise rotation of 13.7° ± 12.2°. This counterclockwise rotation is consistent with Garfunkel's tectonic model for the Mojave block. The paleomagnetic data from southwestern Arizona could indicate that a similar counterclockwise rotation during Basin and Range crustal extension has also occurred in southwestern Arizona.