||Polyhydroxyalkanoates (PHAs) are a family of polyesters formed in bacteria as a carbon or energy storage. PHA production by bacterium Alcaligenes eutrophus from acetic, propionic and butyric acids was investigated in nutrient-rich and chemically defined media by using batch and semi-batch cultures. PHA formation under nutrient-rich conditions was associated with cell growth. High acetate concentrations (5 g/L or above) inhibited both cell growth and PHA formation, but the toxic effect was reduced with increased cell mass. Both cell growth and PHA formation were linearly proportional to acetate concentration in a nontoxic concentration range. By using a semi-batch cultivation, butyric acid was controlled at a low level (20-40 mg/L) and the specific PHA formation rate was linearly proportional to the acid concentration showing no toxic effect of butyric acid in a chemically defined medium. At an ammonium concentration of 80-100 mg/L, butyric acid was used mainly for PHA formation with a little increase in active biomass. Fed with propionic acid, A. eutrophus produced a copolymer, poly(hydroxybutyrate-co-hydroxyvalerate), in contrast to a homopolymer, polyhydroxybutyrate, from acetic or butyric acids. The content of hydroxyvalerate in the copolymer was around 40%.