Several thousand earthquakes were recorded by portable seismic stations operated in Guatemala within the first week following the destructive earthquake of February 4, 1976. The seismically active zone followed the Motagua fault through eastern and central Guatemala, and then branched sharply southward about 35 km west of Guatemala City. These data support the conclusion from field evidence that the main earthquake was caused by movement within the Motagua fault zone. A total rupture length of at least 250 km, defining a segment of the active boundary between the North American and Caribbean plates, is inferred. The southward trending zone of aftershock activity may be a newly defined section of the boundary, or a secondary fault.