We investigate the heteroepitaxial growth of two films of large planar molecules on an inorganic substrate: for the particular system [perylene-tetracarboxylic-dianhydride (PTCDA) on hexa-peri-benzocoronene (HBC) on highly ordered pyrolytic graphite (HOPG)], large domains of several hundred of nanaometers in size with interfaces that are flat and almost defect-free on the molecular scale are observed using scanning tunneling microscopy (STM). The images obtained represent, to the best of our knowledge, the first STM observation of the ordered growth of large planar molecules in a heteroepitaxially stacked film. The mutual lattice alignment and epitaxial relations between the two organic layers are determined from the pronounced moire pattern in the STM images. Beside these findings, the intramolecular contrast of HBC on HOPG is used to determine the molecular orientation of the molecules within the two-dimensional unit cell by STM. (C) 2000 Elsevier Science B.V. All rights reserved.