Abstract Copolymers of ( R)-3-hydroxybutyric acid (( R)-3HB) and ( S,S)-lactide (( S,S)-LA) with a wide range of compositions varying from 5 to 92 mol% ( S,S)-LA were synthesized by the ring-opening copolymerization of ( R)-β-butyrolactone with ( S,S)-lactide at various feed ratios in the presence of 1-ethoxy-3-chlorotetrabutyldistannoxane as a catalyst. The structure and physical properties of P[( R)-3HB- co-( S,S)-LA] were characterized by 1H and 13C n.m.r. spectroscopy, X-ray diffraction, differential scanning calorimetry, and optical microscopy. The copolyesters were shown to have a random sequence distribution of ( R)-3HB and ( S,S)-LA monomeric units. The glass-transition temperature of P[( R)-3HB- co-( S,S)-LA] increased linearly from 4 to 59°C as the ( S,S)-LA fraction was increased from 0 to 100 mol%. The degree of X-ray crystallinity of solvent-cast copolyester films decreased from 62% to 18% as the ( S,S)-LA composition was increased from 0 to 28 mol%, and the copolyesters showed a P[( R)-3HB] crystal lattice. In contrast, the X-ray crystallinities of copolymers with ( S,S)-LA fractions of 70 to 100 mol% increased from 22% to 46% with the ( S,S)-LA fraction, and those samples showed a P[( S,S)-LA] crystal lattice. The P[( R)3HB- co-( S,S)-LA] samples with 47 to 70 mol% ( S,S)-LA were amorphous polymers. Enzymatic degradations of P[( R)-3HB- co-( S,S)-LA] films were carried out at 37°C in an aqueous solution containing PHB depolymerase purified from Alcaligenes faecalis or proteinase K from Tritirachium album. The rates of enzymatic degradation by PHB depolymerase of copolymer films ranging from 5 to 18 mol% ( S,S)-LA were higher than that of microbial P[( R)-3HB] film. The highest rate of enzymatic hydrolysis by PHB depolymerase was observed at 5 mol% ( S,S)-LA. Little erosion was observed for the copolyester films ranging in ( S,S)-LA fractions from 47 to 100 mol%. In contrast, the weight loss profile of P[( R)-3HB- co-( S,S)-LA] films with a proteinase K showed the opposite trend in the copolymer composition to that of a PHB depolymerase. The highest rate of enzymatic hydrolysis by proteinase K was observed at 92 mol% ( S,S)-LA.