The hydrolytic degradation behaviour and some other properties were analysed for blends of microbial poly(3-hydroxybutyrate) [P(3HB)] with microbial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV); 3HV content, 22.3 mol%]. From the analysis of differential scanning calorimetry (d.s.c.) curves, the main component of the crystalline phase in the blend films was found to be P(3HB). The crystallinity of P(3HB) decreases on blending with P(3HB-co-3HV). The hydrolytic degradation of the blend films was carried out on the solution-cast films in phosphate buffer at 55 degrees C and pH 7.4. The rate of hydrolytic degradation of the blend films was accelerated by blending with P(3HB-co-3HV), and showed linear correlation with crystallinity. Therefore, the degradation rate of P(3HB) can be controlled by blending with P(3HB-co-3HV). The lower the initial crystallinity of the blend films, the higher the rate of hydrolytic degradation is seen to be.