From the more than 1500 Mammoth Lakes earthquakes recorded on three-component digital seismographs (Spudich et al., 1981), 150 were used in an analysis of the locations, mechanism, and source parameters. A composite fault plane solution of nine earthquakes 3.9?M?5.1 defines a right-lateral strike slip mechanism on a steeply dipping nearly east-west plane striking S75 ¿E or left-lateral strike slip on a nearly north-south plane striking N10 ¿E. Vertical cross sections of well-located aftershocks indicate possible three east-west planes that coincide with the locations of the four largest earthquakes with ML?6.0. Using the spectral analysis of S waves (Brune, 1970), source parameters for 67 earthquakes were determined. Forty-eight had magnitudes greater than or equal to 3.0. Seismic moments ranges from 9.20¿1018 dyn cm to 2.33¿1024 dyn cm. Earthquakes with seismic moment greater than about 1.0¿1021 dyn cm had nearly constant stress drops (?50 bars); earthquakes with seismic moment less than about 1.0¿1021 dyn cm had stress drop that apparetnly decrease as seismic moment decreases.