The growth of pentacene (Pn) on highly ordered pyrolytic graphite (HOPG) has been investigated by low-energy electron microscope (LEEM). Initially an ordered Pn layer with lying-down molecular orientation is formed on HOPG. The in-plane structure of the first Pn layer corresponds to a modified ac-plane with two different epitaxial orientations and their twining. No island nucleation could be detected in real time LEEM for the epitaxially ordered first layer, which corresponds to the wetting layer. Subsequently, separated nucleations of Pn with both lying-down and standing-up geometries occur on the top of the wetting layer. The lying-down overlayer has the in-plane structure of Pn ac-plane. The lattice parameters and symmetry of the overlayer, which are obtained from in-situ diffraction patterns, reveal a structural difference as compared to the first layer of pentacene. (C) 2009 Elsevier B.V. All rights reserved.