A series of bacterially produced poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) copolymers containing random distributions of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) were characterized by solid-state CPMAS C-13 NMR to determine the degree of cocrystallization of the two components. The 3HV minor component was shown to be incorporated into the poly(beta-hydroxybutyrate)-type crystalline phase over the 3HV composition range of 0-27 mol %. The ratio of the 3HV content in the crystalline phase to the overall 3HV composition increases with increasing 3HV content, implying that it is easier to accommodate the larger 3HV residue in a crystalline region which already contains lattice imperfections. For samples with 3HV compositions of 21 and 27% the 3HV content in the crystalline phase was roughly 2/3 of the overall 3HV content. Asymmetric line broadening in the C-13 MMR spectra showed that the 3HV ethyl side branches assume multiple configurations when included in the crystalline phase. For the 27% 3HV sample, two different crystalline structures were detected-the poly(beta-hydroxybutyrate) (PHB) and poly(beta-hydroxyvalerate) (PHV) type lattice structures. The latter crystal structure is present despite the fact that the overall 3HV composition is below 40%, the composition at which the PHV-type crystal structure is generally formed. Both crystal types are formed because the 3HV content in the noncrystalline phase is above the changeover composition. The coexistence of PHV and PHB lattices suggests that there may also be a change in crystal composition as a function of crystallization time.