The aims of this study were to investigate the reaction kinetics of production o f poly(3-hydroxybutyric-co-3-hydroxyvaleric) acid, P(HB-co-HV), to control its mole fraction in a fermenter, and to develop an optimal copolymer production system using fed-batch cultures of Alcaligenes eutrophus ATCC 17699. In this study, butyric and valeric acids were utilized as carbon sources. In order to keep the concentrations of these fatty acids constant, the well-organized fed-batch experiments were performed using a 5 l jar fermenter equipped with a temperature and pH control system. Mole fraction and sequence distribution of monomeric units in a copolymer, 3-hydroxybutyric acid (HB) and 3-hydroxyvaleric acid (HV), were determined by H-1 NMR and C-13 NMR spectroscopy, respectively. The specific production rate of P(HB-co-HV) was lower than that of PHB when the total fatty acid concentration in the medium was 3.0 g/l, and it was constant and independent of the mole fraction of valeric acid at total fatty acid concentrations less than 0.3 g/l. The mole fraction of HV units in copolymers (0-40%) was linearly related to the mole fraction of valeric acid in the medium. Thus, the mole fraction of HV units in copolymer can be controlled within the range of 0-40 mol% by adjusting the mole fraction of butyric acid in the feeding medium. Moreover, for P(HB-co-HV) with a given mole fraction of HB and HV units, the most economical feeding strategy in terms of achieving the maximum reaction rate involved adjusting the total fatty acid concentration. From the sequence distribution of diads and triads of HB and HV units, P(HB-co-HV) with a 0-40% mole fraction of HV was concluded to be a random copolymer. The weight average molecular weight of P(HB-co-HV) measured by the GPC method changed from 0.77 x 10(6) to 3.4 x 10(6). This range is of the same order of magnitude as the homopolymer, PHB.