The crystal structure has been solved and refined by a combination of three-dimensional Patterson and Fourier series and by the method of least squares. 1873 independent integrated intensities were measured by PEXRAD; the final agreement factor between measured and calculated structure factors is 0.0574. Two crystallographically independent Mn atoms are surrounded by highly distorted oxygen octahedra; the Mn-O distance varies between 2.091 +/- 0.006 and 2.252 +/- 0.003 angstrom, with average length 2.164 angstrom. Two crystallographically independent Mo atoms are surrounded by slightly distorted oxygen tetrahedra, with Mo-O distances ranging from 1.724 +/- 0.005 to 1.851 +/- 0.004 angstrom, the average being 1.761 angstrom. The thermal vibrations are significantly anisotropic. An analysis is given of various current methods for estimating the standard deviation in the measured structure factor.