High molecular mass and high regioregularity favor the more common polymorph (form I) of poly(3-alkylthiophenes) and substantially extend toward high temperatures the existence domain of tile 2D mesomorphic phase, which these systems access above 70-80 degrees C. Morphological observations indicate that in thin films of form I the planar polythiophene main chains lie roughly edge-on parallel to the film surface, while side chains art. approximately orthogonal to the substrate. Evidence relative to the second polymorph (form II), easily obtained with low-molecular-mass material, indicates that sidechain interdigitation is unlikely in this modification, just as in form I. Diffraction patterns in which the two crystalline phases coexist show that the longer axis directions in the two forms are approximately orthogonal, suggesting differences In the crystallization process. Very low-molecular-weight systems tend to crystallize preferentially in form II and give rise to a mesophase characterized by very low degrees of order, probably monodimensional. High regioregularity of the samples appears to affect both the intramolecular order and the packing.