In shake-flask culture, Methylobacterium extorquens accumulated poly(3-hydroxybutyrate) (PHB) possessing a substantially higher weight-average molecular mass (M(w)) than previously reported for this organism. The M(w) of PHB produced by M. extorquens was dependent on the initial concentration of methanol or sodium succinate, used as sole carbon sources. The highest M(w) values (0.6 and 1.7x10(6)) were obtained with low initial concentrations of methanol or sodium succinate (4.0 and 3.0 gl(-1), respectively) and the latter substrate always yielded PHB of higher M(w) than that produced from methanol. Thus PHB with an M(w) in the range 0.2-1.7 x 10(6) could be produced by selection of the carbon source and its concentration. In contrast to the findings with M. extorquens, the M(w) of PHB produced by Alcaligenes eutrophus was high (1.1-1.6x10(6)) and generally unaffected by the choice or concentration of the carbon source. The use of glycerol as sole carbon source did, however, result in the accumulation of PHB with a markedly lower M(w) (5.5-8.5 x 10(5)) than that produced from other sole carbon sources by this organism under similar conditions.