Poly(3-hydroxybutyrate) (PHB) accumulated in Alcaligenes eutrophus H16 when nitrogen nutrient was limited. The addition of nitrogen nutrient ($NH_{4}Cl$) to the medium during PHB synthesis caused the bacteria to cease PHB accumulation, indicating that the synthesis of PHB is regulated by nitrogen assimilation. Accumulated PHB in cells was gradually degraded when the carbon nutrient fructose was exhausted. Degradation of PHB increased when bacteria were transferred to carbon-free media, even in the absence of nitrogen. However, the degradation of PHB in cells was stimulated by the addition of nitrogen nutrient to carbon-free media. The activity of intracellular PHB depolymerase was measured using $^{14}C$-labelled PHB as a substrate. While autodegradation of PHB native granules occurred in vitro, little enzyme activity was detected in the cytosolic fraction, suggesting that most of the enzyme is bound to PHB granules. Autodegradation of PHB was detected at two different pH values, 7.0 and 10.5. Although the effects of inhibitors, such as phenylmethylsulfonyl fluoride and Triton X-100, on autodegradation were similar at both pH values, the effects of dithiothreitol, $MgCl_2$, and bovine serum albumin were different depending on pH.