In situ imaging of the development on lamellar crystals in thin films for poly[(R)-3-hydroxybutyric acid-co-10 mol%-6-hydroxyhexanoic acid] was carried out at a high crystallization temperature of 110 degreesC by temperature-controlled atomic force microscopy. The microfibril crystals with dimensions of 100-500 nm length, 50-90 nm width and 6-8 nm thickness were observed at the growth front of lamellae. These microfibril crystals grew along the longitudinal direction of lamellae during crystallization. In some cases, newly formed microfibril crystal and branching microfibril crystal were observed. (C) 2000 Elsevier Science Ltd. All rights reserved.