The 11 December, 1995 earthquake is the largest and best constrained instrumentally recorded event which has occurred on the Rivera-Cocos plate boundary. The reported focal mechanism for this event indicates almost pure strike-slip faulting with nodal planes oriented north-south and east-west. A visual inspection shows that the seismograms recorded world-wide strongly suggest a directivity effect indicative of a rupture propagating eastward from the epicenter. This observation is confirmed by a directivity analysis which shows a sharp reduction in the difference between observed and synthetic seismograms when the rupture direction is at an azimuth of approximately 90¿. These results indicate that the east-west trending nodal plane of the earthquake of 11 December, 1995 is the actual fault plane. Considering that this is the largest instrumentally recorded earthquake in the region, it strongly suggests that the relative motion of the Rivera plate with respect to the Cocos plate takes place along east-west oriented faults, and that the sense of motion is right-lateral, strike-slip. ¿ 1999 American Geophysical Union