In an effort to improve the strain activity of Alcaligenes eutrophus to produce poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) [P(HB-co-HV)], a mutant strain BK23 was obtained by MNNG mutagenesis. With the strain, the fraction of hydroxyvalerate (HV) and its yield from propionic acid were found to be significantly improved compared to those of the parent strain, A. eutrophus NCIMB 11599. In a fed-batch culture, the mutant strain produced 47 g/l of P(HB-co-HV) with an HV fraction of 22 mol%, while the parent strain could yield only the much lower fraction of 10 mol%.