A block copolymer (P[R,S)-3HB-b-6HH]) of atactic poly[(R,S)-3-hydroxybutyrate] (P[(R,S)-3HB]) and poly(6-hydroxyhexanoate) (P(6HH)) was prepared by ring-opening polymerization of (R,S)-beta-butyrolactone followed by the subsequent polymerization of epsilon-caprolactone in the presence of Zn(C2H5)(2)/H2O catalyst. The block copolymer was an A-B diblock copolymer comprising atactic P[(R,S)-3HB] (A) and P(6HH) (B) segments. The block copolymer was used as a compatibilizer for the immiscible blend of microbial poly[(R)-3-hydroxybutyrate] (P[(R)-3HB]) and P(6HH). The addition of a small amount of the block copolymer into the P[(R)-3HB]/P(6HH) blend reduced the size of the P(6HH) dispersed domains. Good adhesion of the interface between P[(R)-3HB] and P(6HH) was achieved by the intervention of the block copolymer, resulting in an improvement of mechanical properties of the blends. The enzymatic degradation of blend films was carried out at 37 degrees C and pH 7.4 in a 0.1 M phosphate solution of an extracellular PHB depolymerase from Alcaligenes faecalis T1. The enzymatic degradation took place on the surface of the blend films, and the erosion rate decreased in the following order : P[(R)-3HB]/P(6HH) [75/25 (wt %)] > P[(R)-3HB]/P(6HH)/P[(R,S)-3HB-b-6HH] [71/24/5 (wt %)] > P[(R)-3HB] >> P(6HH).