X-ray diffraction studies on poly(ferrocenylenedimethylsilylene), PFDMS, indicate a 3D crystalline phase with monoclinic packing of the macromolecules with possible unit cell parameters a = 13.29 Angstrom, b = 6.01 Angstrom, c = 13.9 Angstrom, and gamma = 93.6 degrees. There are two main chains with four monomer units per unit cell, X-ray crystal density rho(c) = 1.455 g cm(-3), possessing a planar all-trans zigzag conformation of the main chain, helix 2/1. Poly(ferrocenylenedibutylsilylene), PFDBS, also appears to crystallize in a monoclinic lattice with unit cell parameters a = 11.57 Angstrom, b = 6.71 Angstrom, c = 13.6 Angstrom and gamma = 107.1 degrees (two monomer units of one macromolecule per unit cell; X- ray crystal density rho(c) = 1.239 K cm(-3)). A particular feature of crystalline PFDMS, and PFDBS, is the coexistence of the 3D monoclinic crystalline phases with mesophases (2D crystals with a hexagonal or tetragonal packing of macromolecules). Upon doping with iodine vapor, the crystalline poly(ferrocenylenedimethylsilylene) converts into a crystalline mixed-valence complex. Doped oriented samples retain the initial orientation of the macromolecules and also give relatively good fiber diffraction patterns. Equatorial reflections on such fiber patterns can be indexed by a 2D monoclinic packing of the complex macromolecules in the (ab) plane with unit cell parameters a = 10.36 Angstrom, b = 6.03 Angstrom, and gamma = 90.7 degrees. The values of the a and b dimensions suggest that [I-3](-) species are placed between macromolecules and along their backbone. Three meridional reflections at 3.0, 4.0, and 6.0 Angstrom yield a fiber repeat equal to 12.0 Angstrom, which probably arises from a one-dimensional order of [I-3]arrays.