Abstract: A new poly-.beta.-hydroxy alkanoate (PHA) copolymer is produced in the cell of Pseudomonas cepacia KYG-505 (KCCM 10004) and a variety of polymer blends of PHB and PHA are developed, The PHA copolymer can improve the properties of known PHB through blending and, having double bonds on some of its side chains, exhibits capabilities for organic synthesis, whereby it useful in production of polymers with various properties and biodegradability.