Biodegradable copolyesters of 3-hydroxybutyrate-co-3-hydroxyvalerate (3HB-3HV) were produced by Alcaligenes eutrophus in a two-staged process, namely growth stage and nitrogen-deficient polyester-accumulation stage. When C-5 was used as the sole carbon source, the copolyester contained 43 mol % of 3HV. A range of copolyesters with 0-43 mol % of 3HV could be produced by using a medium containing different concentration ratios of butyric acid C-4 and C-5. Tm of PHB homopolymer was 177.6 degrees C and that of copolyester with highest 3HV mol fraction of 43% was 99.0 degrees C. C-5 concentration in the medium could be an effective means to control the polymeric composition and mechanical properties of the copolyesters accumulated in A. eutrophus.