Copolyesters of 3-hydroxybutyrate (3HB) and 3-hydroxypropionate (3HP) with a wide range of compositions varying from 0 to 88 mol % 3HP were produced by Alcaligenes latus from the mixed carbon substrates of 3-hydroxypropionic acid with sucrose or 3-hydroxybutyric acid. The copolyesters were shown to have a random sequence distribution of 3HB and 3HP monomeric units by analysis of C-13 NMR spectra. The number-average molecular weights of the copolymers were in the range (1.1-3.5) X 10(5). The structure and physical properties of P(3HB-co-3HP) copolymers were characterized by H-1 and C-13 NMR spectroscopy, X-ray diffraction, gel permeation chromatography, differential scanning calorimetry, dynamic mechanical spectroscopy, and dielectric spectroscopy. The crystallographic parameters of copolymers with compositions up to 43 mol % 3HP were little influenced by the presence of the 3HP unit, but the X-ray crystallinities decreased with the 3HP fraction. The isothermal radial growth rates of spherulites of P(3HB-co-11% 3HP) were slower by 2 orders of magnitude than the rate of P(3HB). The rate of enzymatic degradation of P(3HB-co-3HP) films increased with an increase in the 3HP fraction.