The large 50S subunit of the Haloarcula marismortui 70S ribosome was solved to 19 Angstrom using cryoelectron microscopy and single particle reconstruction techniques and to 9 Angstrom using X-ray crystallography In the latter case, phases were determined by multiple isomorphous replacement and anomalous scattering from three heavy atom derivatives. The availability of X-ray and electron microscopy (ERI) data has made it possible to compare the results of the two experimental methods, In the flexible regions of the 50S subunit, small differences in the mass distribution were detected. These differences can be attributed to the influence of packing in the crystal cell. The rotationally averaged power spectra of X-ray and EM were compared in an overlapping spatial frequency range from 60 to 13 Angstrom. The resulting ratio of X-ray to EM power ranges from 1 to 15, reflecting a progressively larger underestimation of the Fourier amplitudes by the electronmicroscope.