The crystalline structure and semicrystalline morphology of poly(di-n-propyl siloxane), PDPS, were studied with powder and fiber X-ray diffraction, differential scanning calorimetry, solid-state nuclear magnetic resonance and atomic force microscopy (AFM). PDPS exhibits two crystalline phases, alpha (-45.5 degrees C < T < 62 degrees C) and beta (T < -45.5 degrees C), and a hexagonal columnar mesophase which is stable in a broad temperature range between 62 and 197 degrees C. The low-temerature crystalline phase beta is found to be monoclinic with lattice parameters a = 20.60 angstrom, b 19.22 angstrom, c = 4.95 angstrom, gamma = 93.1 degrees. The alpha crystalline phase has a pseudo-tetragonal unit cell with a = b = 19.15 angstrom and c = 5.00 angstrom and is characterized by monoclinic C2/c (No. 15) group symmetry. In the refined unit cell (R-wp = 0.127) the PDPS chains adopt a planar cis-trans conformation with the plane of the chain parallel to the b axis. The semicrystalline morphology of the alpha crystal was studied with AFM. The crystallization of PDPS from the hexagonal mesophase results in very thick (100-150 nm thick) crystalline lamellae, which implies that the chains are fully extended in the crystal. This feature, which is found in few polymers, makes PDPS similar to such systems as HDPE at high pressure or 1,4-trans-poly(butadiene).