The kinetics of crystallization induced by orotic acid (OA) and boron nitride (BN) as nucleating agents were investigated for bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (P(HB-co-HH)s) containing from 0 to 18% HH monomer units. The nucleation efficiency of these two chemicals was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). It was found that both orotic acid and boron nitride are able to nucleate the crystallization of PHB. In the case of P(HB-co-HH) copolymers, orotic acid showed an outstanding nucleating effect. The comparison of half-crystallization times shows that for P(HB-co-10% HH), the crystallization initiated by orotic acid is more than three time faster than the one induced by boron nitride (t(1/2BN)/t(1/2OA)(60 degrees C) = 3.7 and t(1/2BN)/t(1/2OA)(90 degrees C) = 4.5). According to the fact that orotic acid is a biodegradable, biocompatible and a nontoxic chemical, this nucleating agent is a promising solution for PHAs used in medical applications such as implants. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:1287-1294, 2009