Late Cretaceous and Eocene paleomagnetic results from calcareous mudstone, limestone, and volcaniclastic rocks in southwest Puerto Rico yield a significant anticlockwise discordance in magnetization direction relative to North American reference data. Progressive thermal demagnetization has revealed stable, characteristic high blocking temperature magnetization components that display an intrasite bipolarity. The direction of this stable magnetization in tilt-corrected coordinates is consistently either northwest and down, or southeast and up. The pole position (39.4¿ N.Lat.; 206.6¿ E.Long.; dp=9.3¿; dm=16.7¿; N=10) suggests post Eocene anticlockwise rotation of the sampling region relative to North America by 45¿ about a vertical axis. A rotating block domain model in which north-south compression is superimposed on WNW-ESE strike-slip faults in SW Puerto Rico can explained the inferred rotation. ¿ American Geophysical Union 1989