Directions of paleomagnetism in dikes and flows in Miocene age on Guam (lat. 13.3° N., long. 144.7° E.) deviate considerably but systematically from the present Earth field direction. The mean inclination of the eight sites is 24° (downward), which is nearly exactly that of the axial dipolar field for this latitude; but the mean declination lies 55° clockwise from true north (α95 = 21°). A simple explanation is that the southernmost portion of the Mariana island arc has been tectonically rotated about a vertical axis in a clockwise direction about 50° to 60° since Miocene time but has undergone little, if any, latitudinal shifting. These conclusions are also consistent with available interpretations of seismic data for the southern Mariana arc which suggest components of right-lateral strike slip as well as thrust movement along the generally east-west zone separating the Philippine Sea plate on the north from the Pacific plate on the south.