The poly(3-hydroxybutyrate-co-3-bydroxyvalerate) (PHBV)/montmorillonite (MMT) nanocomposites were investigated by wide-angle X-ray scattering (WAXS). The aim of the investigation was solution intercalation of MMT with PHBV Beside the usual orthorhombic unit cell, a stable pseudohexagonal beta-structure of PHBV was obtained. Well known beta-structure has one common WAXS reflection (d = 0.480 nm), which corresponds to the mean distance of PHBV chains in the pseudohexagonal structure. The new beta-structure has two diffraction peaks in the WAXS pattern. It is a three-dimensionally ordered crystalline structure oriented in parallel with the silica layers of MMT. The new polymorphic form is supposed to be growing on the layers of MMT. Its layers serve as primary nucleation centers for epitaxial growth of the beta-structure. After annealing, this polymorphic form of PHBV disappears and it is transformed into the more stable alpha-form leading to an enhanced total crystallinity of the polymer comprised in the nanocomposite. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 751-755, 2009