Polyhydroxybutyrate (PHB) is a biodegradable bacterial polyester emerging as a viable Substitute for synthetic, semicrystalline, nonbiodegradable polymers. An elastomer terpolymer of acrylonitrile-g-(ethylene-co-propylene-co-diene)-g-styrene (AES) was blended with PHB in a batch mixer and in a twin-screw extruder to improve the mechanical properties of PHB. The blends were characterized with differential scanning calorimetry, dynamic mechanical analysis, scanning electron microscopy, and impact resistance measurements. Despite the narrow processing window of PHB, blends with AES could be prepared via the melting of the mixture without significant degradation of PHB. The blends were immiscible and composed of four phases: poly(ethylene-co-propylene-co-diene), poly(styrene-co-acrylonitrile), amorphous PHB, and crystalline PHB. The crystallization of PHB in the blends was influenced by the AES content in different ways, depending on the processing conditions. A blend containing 30 wt %. AES presented impact resistance comparable to that of high-impact polystyrene, and the value was about 190% higher than that of pure PHB. (c) 2008 Wiley Periodicals, Inc.