Poly(ethylene glycol) of M(n), 200 g/mol (PEG-200) was added to cultivation media of Alcaligines eutrophus during polymer formation, and the carbon source used was 4-hydroxybutyrate. The addition of PEG-200 from 0 to 2 wt % resulted in increased 4-hydroxybutyrate (4HB) incorporation from 66 to 86 mol % into the poly(3-hydroxyalkanoate) (PHA), formed. When 4% PEG was used, the mole percent of 4HB in PHA decreased to 64 mol %. Thus, PEG in cultivation media resulted in polyester compositional changes. An unexpectedly large amount of the dry cell weight (similar to 10%) was found to be PEG-200 for cultivation media amended with 4% PEG-200. Analysis of this product by gel. permeation chromatography (GPC) showed it to be complex, having multiple peaks. The repeat unit sequence distribution determined by 75 MHz C-13 NMR spectroscopy indicated that the addition of PEG-200 to media resulted in the formation of a product with predominantly 3-hydroxybutyrate (3HB)-3HB and 4HB-4HB diads instead of random copolyesters which are formed in the absence of PEG addition to media. Two dimensional homonuclear (H-1) correlated NMR analysis along with a number of control experiments indicates that PEG was covalently linked at the carboxy terminal position of PHA chains, forming natural-synthetic diblocks. Fractionation of this product into acetone soluble (AS, 84 mol % 4HB, M(n) = 37 400 g/mol) and insoluble (AIS, 95 mol % 3HB, M(n) = 130 000 g/mol) fractions was carried out which proved that the product formed is a mixture of polyesters differing in composition and molecular weight. Analysis of the unfractionated product as well as AS and AIS fractions by differential scanning calorimetry supported the above conclusions.