The biosynthesis of poly (beta-hydroxybutyrate) (PHB) by Bacillus circulans was carried out through cultivation in nutrient broth in a single step. <(M)over bar v> of polymer fractions soluble in chloroform were close to 2 x 10(5). IR, H-1 and C-13 NMR spectra of the isolated PHB were in agreement with the assumed structure of this poly-beta-hydroxyester. NMR spectroscopy pointed out a high percentage of isotactic polymer, while differential scanning calorimetry showed that the PHB was crystalline.