The end groups of bacterial poly(3-hydroxybutyrate) (PHB) were analyzed by H-1 and P-31 NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A PHB component having a molecular weight around 200 000 Da was broken down into low-molecular-weight oligomers by acid-catalyzed partial hydrolysis. MALDI-TOF MS showed that one series of fragments contain a common terminal group that originated from the primer used for initiating in vivo polymerization. Analysis of the fragments proved that the terminal group is succinate. On the basis of this result, a plausible mechanism is proposed for enzymatic polymerization in microbial PHB synthesis.