The crystal structure of dimethyl-oligothiophenes grown by epitaxial crystallization on highly oriented PTFE thin films has been studied by transmission electron microscopy. The dimethyl-oligothiophenes crystallize principally in a monoclinic phase with common lattice parameters a, b, and beta, but with different c for different numbers of thiophene rings : a = 0.598 nm, b = 0.789 nm, and beta = 98 degrees; and c = 1.866 nm for the tetramer, c = 2.234 nm for the pentamer and c = 2.596 nm for the hexamer. The space group is C2/m. The calculated crystallographic densities are 1.36 g/cm(3), 1.39 g/cm(3) and 1.40 g/cm(3) for the tetramer, pentamer and hexamer respectively. Polymorphism is found, with coexistence of monoclinic and orthorhombic crystal structures. The dominant phase, however, is monoclinic. The relationship between the different phases is discussed.