The seismicity of the Isthmus of Tehuantepec, Mexico is anomalously active compared with other parts of the Middle American subduction zone. The results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96¿W, to a dip of about 45¿ to 50¿ to the east of this longitude. The change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km. This geometry of the slab is reminiscent of that in southern Peru; the change in dip, however, takes place more gently in Tehuantepec. In addition to the change in the dip of the subducted Cocos plate, a rapid increase is observed in the maximum depth extent of the subducted slab. In the region of subhorizontal subduction, focal depths are consistently shallower than about 80 km, whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle. Both the change in the dip of the slab and of the maximum depth of intermediate-depth earthquakes appear to correlate with the presence of the Tehuantepec Fracture Zone which separates two distinct provinces of the oceanic Cocos plate. The age and the crustal thickness of the Cocos plate increase to the southeast of the Tehuantepec Fracture Zone relative to its northwestern counterpart. In Tehuantepec, the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate, as has been observed in other subduction zones of the world. ¿ American Geophysical Union 1992