in shaking flask cultures at different rotation speeds or energy dissipation rates. The polymers included poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), poly(epsilon -caprolactone) (PCL), polystyrene (PS), two binary PHBV/PCL blends (80/20 and 25/75 w/w), and a triple PHBV/PCL/PS blend (76/5/19w/w/w). The specific degradation rate of PHBV found from the specimen's residual mass fraction with time was constant after a lag phase and was significantly affected by the agitation strength (<0.5 day(-1) at 60 rpm or lower and >15 day(-1) at 120 rpm or greater). Tiny polymer fragments were formed on the specimen surface and observed with scanning electron microscopy during degradation. The detachment of those fragments under high hydraulic shear stress caused surface erosion and renewal, resulting in the high degradation rate. The hydraulic shear stress (0.6 Pa) at an energy dissipation rate of 0.5 W/kg was a threshold level, above which the external force did not increase the degradation rate very much. PHBV degradation in the binary blends with compatible PCL was retarded, depending on the blend composition. Blending PHBV with noncompatible PS did not affect PHBV degradation, and the overall degradation rate of the triple blend was faster than the rate of PHBV alone because of the surface erosion of both PHBV and nondegradable PS fragments from the specimens.