The continental lithosphere of Mexico south of the Trans-Mexican volcanic belt may consist of two lithospheric blocks, the Jalisco and Southern Mexico Blocks. The existence of these two blocks can be examined employing the statistical F test (formulated to test for the presence of significant misclosure around a plate circuit) to plate motion data derived from marine magnetic anomaly lineations, earthquake slip vectors, and transform fault azimuths. The result of the F test applied to the Pacific-Cocos-North American plate circuit indicates that there is no significant (at the 99% confidence level) misclosure around this plate circuit. Therefore if the Southern Mexico Block exists, its motion relative to the surrounding lithospheric plates is too small to be resolved from these data. In contrast, the result of the F test applied to the Rivera-Pacific-North American plate circuit indicates the presence of a significant misclosure about the Rivera-Pacific-North American plate circuit, the cause of which is uncertain. One interpretation of the misclosure amount the Rivera-Pacific-North American plate circuit is that it is due to the presence of an independent Jalisco Block. However, it is more likely that this misclosure is, instead, primarily due to the effects of recent changes in the relative motion between the Pacific and Rivera plates. Assuming the second explanation is correct, a new Rivera-North American Euler pole location (21.8¿N, 110.4¿W) is determinable solely from earthquake slip vectors located along the Rivera-North American boundary. ¿ American Geophysical Union 1994