Pseudomonas sp. 61-3 isolated from soil was found to produce a polyester consisting of 3-hydroxyalkanoic acids of even carbon numbers C4, C6, C8, C10 and C12 when sodium gluconate was fed as the sole carbon source. The polyester produced was fractionated with boiling acetone. The acetone-insoluble fraction (28 wt%) of the polyester was a poly(3-hydroxybutyrate) homopolymer, while the acetone-soluble fraction (72 wt%) was composed of seven different 3-hydroxyalkanoate (3HA) units ranging from C4 to C12: 40 mol% 3-hydroxybutyrate (3HB), 5 mol% 3-hydroxyhexanoate (3HH), 20 mol% 3-hydroxyoctanoate (3HO), 24 mol% 3-hydroxydecanoate (3HD), 1 mol% 3-hydroxy-5-cis-decenoate (3H5D), 4 mol% 3-hydroxydodecanoate (3HDD) and 6 mol% 3-hydroxy-5-cis- dodecenoate (3H5DD). The copolymer was characterized by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography and differential scanning calorimetry. The acetone-soluble fraction of this amorphous copolymer was shown to have a random sequence distribution of the seven 3HA units of C4 to C12 by analysis of the 150 MHz 13C-NMR spectrum. This is the first example of microbial synthesis of a random copolyester consisting of 3HB and medium-chain-length 3HA units.