The three-dimensional growth of para-sexiphenyl (p-6P) on KCl(001) is characterized by a large polymorphism. Using grazing-incidence diffraction, we have found that two of the known five orientational phases feature preferential in-plane alignments along the [110] in-plane direction of the substrate: p-6P(11-1) parallel to KCl(001) with [1 - 10] parallel to KCl[110] as well as p-6P(001) parallel to KCl(001) with [1 -10] parallel to KCl[110]. The latter structure is a peculiar low-symmetry configuration, which can be understood by analysis of the molecular crystal lattice and the substrate symmetry. (C) 2002 Elsevier Science B.V. All rights reserved.