Among the various biomaterials available for tissue engineering and therapeutic applications, microbial polyhydroxyalkanoates offer the most diverse range of thermal and mechanical properties. In this study, the biocompatibility of poly(3‐hydroxybutyrate‐co‐4‐hydroxybutyrate) [P(3HB‐co‐4HB); containing 50 mol % of 4‐hydroxybutyrate] copolymer produced by Delftia acidovorans was evaluated. The cytotoxicity, mode of cell death, and genotoxicity of P(3HB‐co‐4HB) extract against V79 and L929 fibroblast cells were assessed using MTT assay, acridine orange/propidium iodide staining, and alkaline comet assay, respectively. Our results demonstrate that P(3HB‐co‐4HB) treated on both cell lines were comparable with clinically‐used Polyglactin 910, where more than 60% of viable cells were observed following 72‐h treatment at 200 mg/mL. Further morphological investigation on the mode of cell death showed an increase in apoptotic cells in a time‐dependent manner in both cell lines. On the other hand, P(3HB‐co‐4HB) at 200 mg/mL showed no genotoxic effects as determined by alkaline comet assay following 72‐h treatment. In conclusion, our study indicated that P(3HB‐co‐4HB) compounds showed good biocompatibility in fibroblast cells suggesting that it has potential to be used for future medical applications. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 2007