The genes for biosynthesis of the biodegradable polymer poly-beta-hydroxybutyric acid (PHB) cloned from Alcaligenes eutrophus H16 were used for synthesis of PHB with recombinant Escherichia coli strains. It was recognized that the PHB-biosynthesis genes cause segregational instability to the plasmids used as vectors. Recombinant PHB-plasmids are rapidly lost from host cells and plasmid-free cells occur at high rates, even under conditions of selection for the plasmids. Cloning the partitioning region of plasmid RP4 onto such plasmids resulted in a high degree of stabilization. These par-stabilized recombinant PHB-plasmids could be maintained quite efficiently in batch cultivation experiments in the absence of any selection pressure.