Hypocenters and magnitudes of more than 600 aftershocks of the 1966 Parkfield-Cholame earthquake were determined from recordings of a dense network of portable seismograph stations operated in the epicentral region from 3 to 82 days after the main shock. Hypocenters were virtually confined to a nearly vertical zone extending downward from the zone of visible ground fracturing at the Earth's surface to a depth of 12 to 14 km. Aftershocks were concentrated in patches on the slip surface, with large numbers of events in depth ranges of 2 to 4 km and 8 to 10 km and relatively few at depths of 5 to 7 km. First-motion patterns suggest that simple, nearly horizontal right-lateral strike-slip displacement was the source for an overwhelming majority of the aftershocks. The coefficient “b” in the frequency versus magnitude equation appears to be depth-dependent: it is about − 0.6 for events between 8- and 10-km depth but averages about −0.95 for events at other depths. Individual stations with consistently late P-wave arrivals were also found to record abnormally large amplitudes, with both anomalies increasing, apparently, with increasing thickness of sediments beneath the site.