The crystal structure of poly(4-hydroxybutyrate) (P(4HB)) has been determined by interpretation of X-ray fiber diagrams of a stretched-annealed film and electron diffraction patterns of lozenge-shaped single crystals prepared from ethanol solution. The unit cell of P(4HB) is orthorhombic with space group P2(1)2(1)2(1) and parameters a = 0.775 +/- 0.002 nm, b = 0.477 +/- 0.002 nm, and c (fiber axis) = 1.199 +/- 0.002 nm. There are two chains per unit cell, which exist in an antiparallel arrangement. The chain conformation has been analyzed through energy minimization by using the Cerius(2) software. The helical conformation is characterized as a slightly distorted all-trans conformation. Molecular packing of this structure has been studied in detail taking into account both diffraction data and energy calculations. Setting angles, with respect to the b axis, are +/-29degrees for the corner and center chains according to intensity measurements and structure factor calculations. Films and solution-grown lamellar crystals of P(4HB) were enzymatically degraded with lipase from Pseudomonas sp. and PHB depolymerase from Pseudomonas stutzeri YM1006. The enzymatic degradation of solution-cast, stretched, and stretched-annealed films of P(4HB) with lipase was investigated by means of weight-loss profiles and scanning electron microscopy. Furthermore, the enzymatic degradation of P(4HB) single crystals with lipase and PHB depolymerase was studied by using transmission electron microscopy, atomic force microscopy, and gel permeation chromatography. All the smooth edges of the lozenge-shaped lamellar crystals became rough after enzymatic degradation. In the meanwhile, no obvious changes of molecular weights and lamellar thickness were observed. Therefore, it is concluded that enzymatic degradation of the crystal region progressed from the crystal edges rather than the chain-folded surfaces.