A major earthqueke, Ms=7.6, occurred off the southern coast of Mexico near the twon of Petartlan on 14 March 1979. Data recorded just before, during, and just after the event (a subset of a large data set extending from six weeks before to four weeks after the event) by local recorders including wide-dynamic-range instruments indicates that the earthquake ruptured a 50-km-long segment of the Middle American subduction zone, closely filling a seismic gap which was last ruptured by a major earthquake (Ms=7.5) in 1943. Since recurrence times for large events in this general area are highly variable, the area remains one of high seismic risk, and provides a focus for continuing investigation of the segmentation in the subduction process which is implied by the seismic gap hypothesis.