Thin films with average layer thickness 100 nm of poly[(R)-3-hydroxybutyric acid] (P[(R)-3HB]) and poly[(R)-3-hydroxybutyric acid-co-10 mol% 6-hydroxyhexanoic acid] were isothermally crystallized at a given crystallization temperature after melting at 200 degrees C. The spherulitic and lamellar morphologies of polyester thin films were investigated by means of optical microscopy, transmission electron microscopy and atomic force microscopy. After isothermal crystallization, the uniform two-dimensional spherulites which have the stacking texture of flat-on lamellae were developed throughout both polyester thin films. Each flat-on crystal gave a well-resolved electron diffractogram, and all electron diffractograms were identical. Accordingly, the 6-hydroxyhexanoic acid units were rejected from the tightly packed region of lamellar crystals. Both the lamellar periodicity and lamellar width of polyester thin films increased with a rise in the crystallization temperature. At the growth front of crystalline lamellae, microfibril crystals with 30-50 nm width were present. The enzymatic hydrolysis of thin films was carried out at 25 degrees C using an aqueous solution of PHB depolymerase from Alcaligenes faecalis. After enzymatic degradation, the jagged texture along the crystal long-axis could be observed at the ends of crystalline lamellae on the surface of the thin films. These observations suggest that the lamellar crystals of P[(R)-3HB] is composed of both tight molecular packing regions of microfibril crystals and loose molecular packing boundary regions.