Spectral amplitudes for 24 events of the Mammoth Lakes earthquake sequence have been determined for the frequency range 0.1--10.0 Hz. The ML6 earthquake that occurred at 1450 UT on May 27, 1980, is of particular interest because of the controversy surrounding its mechanism. A comparison of spectral levels determined from analysis of locally recorded strong motion and broadband digital data and spectral levels from regionally recorded surface waves extrapolated back to the source yields consistent results, indicating flat spectra in the band 0.1--1.0 Hz. The spectra observed in this study do not show pronounced spectral peaks predicted by theoretical studies of ground motion due to the jerky extension of a fluid-driven tensile crack containing a low-viscosity fluid. We have found nothing in the spectra of the ML 6 events that occurred at 1450 UT, on May 27, 1980, nor in the spectra of the aftershocks to distinguish them from spectra of ''tectonic'' earthquakes. However, the spectra themselves do not distinguish between various explanations for the non-double-couple source mechanism observed in moment tensor inversion and first motion data for the largest events of the Mammoth Lakes earthquake sequence.