IDA records of the 19 Sept. 85 Michoacan earthquake and its large aftershock of 21 Sept. 85 have been used to estimate source-mechanism spectra EL Mˆ(&ohgr;) and total-moment spectra MT(&ohgr;) in 1-mHz bands over the frequency interval 1-11 mHz. Source mechanisms, obtained by the phase-equalization method of Riedesel and Jordan, show no significant frequency dependence and no significant non-double-couple components. The best average double-couple solution has a strike, dip and rake of 289¿¿4¿, 19¿¿15¿ and 76¿¿5¿ for the main event, and 293¿¿3¿, 24¿¿12¿ and 73¿¿4¿ for the aftershock. Total-moment spectra, obtained by the power-spectral method of Silver and Jordan, are parametrized by a total integrated moment MT0 and a characteristic time &tgr;c. For the main event, we obtain MT0=10.7¿2.0)¿1020 N-m and &tgr;c=49¿7 s; the corresponding parameters for the aftershock are MT0=(2.6¿0.6)¿1020 N-m and &tgr;c=30¿11 s. Both events are relatively slow; in particular, the aftershock has a larger characteristic time than either the fast-rupturing 29 Nov. 78 Oaxaca or the 14 Mar 79 Petalan earthquakes, although the static moments are about the same. Our results support the model of the main shock as a multiple-rupture event with moment release occuring as long as 100 s after the initial break.