Crystalline/crystalline binary blend films of microbial polyesters composed of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (P(3HB-co-3HH)) and poly[(R)-3-hydroxybutyrate] (P(3HB)) that exhibit a morphological change are prepared by solvent casting. Differential scanning calorimetry measurements indicate that P(3HB-co-3HH) and P(3HB) are miscible for all blend ratios because a single glass transition temperature is observed. Polarization optical microscopy is used to investigate the transition of spherulite morphology and measure the radial growth rate of spherulites in the blend films. P(3HB-co-3HH) and P(3HB) contain positive spherulites, whereas in the binary blends, spherulite morphology changes from positive to negative. This change is related to the different growth rates of P(3HB-co-3HH) and P(3HB) lamellar crystals. Partial enzymatic degradation of the film surfaces reveals that the lamellar crystals of negative spherulites are oriented both perpendicular and parallel to the radial direction of spherulites. A new growth mechanism for spherulites in crystalline/crystalline blends is constructed from the results obtained for the blend films. (C) 2013 Elsevier Ltd. All rights reserved.