The Dulce, New Mexico, earthquake of 23 January 1966 (mb = 5.5, USCGS, mbLg = 5.1, MS = 4.6) occurred as normal faulting on a north-northwest- or north-striking fault at a focal depth of 3 km. The seismic moment of the event was 3.5 × 1023 dyne-cm; source dimension and stress drop are estimated using Brune's (1970) relation as 2.9 km and 6 bars, respectively. The focal mechanism was determined by fitting P-wave first motions, judged to be unambiguous, as well as surface-wave amplitude and phase spectra. Complete theoretical long-period waveforms computed for our focal mechanism and depth agree well with waveforms observed at stations ALQ and GOL. The hypocenter of the main shock and of aftershocks occurring within 31 days of the main shock have been relocated by the method of joint hypocenter determination with respect to the nuclear explosion “Gasbuggy.” The relocated main shock occurred at 36.98°N, 107.02°W at 01:56:38.1 UTC with a focal depth of 3 km. The location and focal mechanism of the main shock point to it having occurred as additional movement on one of a family of north-northwest-striking normal faults that had their maximum activity in Miocene time. The distribution of aftershock epicenters suggests that the main shock triggered aftershock activity on adjacent faults. The focal mechanism agrees with previous inferences that the region of Dulce is currently experiencing tensional tectonic stress oriented approximately eastwest.