The kinetics of poly-D(-)-3-hydroxybutyric acid (PHB) production was studied in Alcaligenes eutrophus H16 (ATCC 17699) fed-batch culture using butyric acid as a sole carbon source. In order to control the butyric acid concentration at a steady value, a constant butyric acid feeding policy was developed. At a butyric acid concentration of 3 g/l, pH 8.0, the maximum values of specific PHB production rate, production yield, and PHB content in the cell were obtained. The effects of the cultivation conditions on the molecular weight distribution of PHB were also studied. The average molecular weight distribution decreased as the butyric acid concentration increased. When a nitrogen source was added to the fermentation medium after adequate PHB accumulation in the cells, not only PHB production but also degradation occurred simultaneously. The degradation rate of low molecular weight polymer was greater than that of high molecular weight polymer, and as a result the average molecular weight shifted to the higher value.