Aftershocks of the 14 March 1979, Petatlan, Mexico, earthquake (MS = 7.6) were recorded by a temporary seismic network deployed by the Hawaii Institute of Geophysics. This paper describes the aftershock activity in the first 54 hr following the main shock. The aftershock area expanded to about 4800 km2 at the end of the first day and did not significantly increase during the second day. Only an additional 26 per cent enlargement occurred by the 36th day. The aftershock region contains two zones of relatively higher seismicity. The ratio of slip between these two zones alternates (1.75, 0.72, 1.73) for time periods determined by the minima and maxima of the slip difference between them. The total slip during the first 54 hr is about 3 mm. The aftershock b value for the rupture region is 1.49, higher than the foreshock b value of 1.11 obtained by the same local network. This result is consistent with the lowered state of stress of the area after the major shock.