We report on the growth and morphology of alpha-quaterthiophene and alpha-sexithiophene clusters, needles (or "fibers"), and flat islands on muscovite mica (001). Samples are prepared by organic molecular beam deposition, and characterized ex situ by polarized fluorescence microscopy and by atomic force microscopy. The growth mechanism of such thiophene fibers is compared to that of para-phenylenes. Distinct needle orientations are present on the surface, their formation being explained by a combination of epitaxy and electric field assisted alignment. (C) 2009 Elsevier B.V. All rights reserved.