Miocene-Oligocene rocks of the Mogollon-Datil volcanic field in southwestern New Mexico have yielded paleomagnetic data giving 61 site-mean/unit VGPs and a paleomagnetic pole located at 81.9¿N, 143.6¿E (A95=4.5¿; k=17.6). This pole is indistinguishable from other mid-Tertiary reference poles for North America. Combining the Mogollon-Datil data with VGP data from other studies of mid-Tertiary (20--40 Ma) rocks yields a paleomagnetic pole at 81.5¿N, 147.3¿E (N=239; A95=2.4¿; k=15.4). When divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95% confidence level . The paleopole determined from reversely magnetized units, located at 78.6¿N, 138.5¿E (N=125; A95=3.4¿; k=14.5), is 6.8¿ on the far side of the paleopole determined from normally magnetized units, positioned at 84.2¿N, 166.4¿E (N=114; A95=3.3¿; k=17.0), when viewed from a common site location (i.e., the western United States). Similar pole positions and differences are also seen in 10-Ma subsets of this data set. Incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results. Rather, we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term et al. 1979>. ¿ American Geophysical Union 1988