The present invention is a method for controlling biopolymer synthesis by determining the genetics and enzymology of polyhydroxybutyrate (PHB) biosynthesis at the molecular level. The purified enzymes and genes provide the means for developing new PHB-like bicpolymers having polyester backbones. Specific aims are to 1) control the chain length of the polymers produced in fermentation processes through genetic manipulation, 2) incorporate different monomers into the polymers to produce co-polymers with different physical properties, and 3) examine the physical/rheological properties of these new biopolymers in order to develop further design criteria at the molecular level.The method for engineering biopolymer synthesis includes: isolation and characterization of the genes for the enzymes in the synthetic pathway (beta-ketothiolase, acetoacetyl-CoA reductase and PHB synthetase); cloning of the genes in a vector(s); placement of the vector(s) under the control of regulated promoters; expression of the genes; determination of the function and use of other factors such as substrate specificity in polymer production and composition; and isolation and physical and chemical analysis of the resulting polymers.