The thermal behavior and physical structure of atactic poly(methyl-n-propylsilane) (PMPrS) have been investigated by complementary techniques. Temperature-dependent wide-angle X-ray scattering as well as thermal analysis clearly indicate that atactic PMPrS crystallizes below 40 degrees C in a monoclinic lattice with PMPrS adopting an all-trans planar zigzag conformation. Above 40 degrees C, the polymer is in the isotropic amorphous state. A restricted analysis of the structure factors of PMPrS has been performed, indicating that the zigzag planes most probably lie in ((1) over bar 10) planes. The chains pack with little interpenetration, and the crystals may be considered as bundles of long, closely packed prisms. The restricted interlocking of neighboring chains results, in turn, in a poor register of the chains along the c-axis. Moreover, transmission electron microscopy reveals that the crystallized polymer adopts a lamellar microstructure, with parallel lamellae tending to form tight bundles. Both electron microscopy and small-angle X-ray scattering indicate crystal thicknesses of about 60 Angstrom. Finally, PMPrS was found to crystallize with a nucleation-controlled type of kinetics. Avrami exponents were calculated as n approximate to 1, suggesting a fibrillar growth geometry compatible with the absence of spherulitic superstructure. A double-melting behavior is also observed for PMPrS.