We tested the synthesis and in vitro activity of the poly(3-hydroxyalkanoate) (PHA) polymerase 1 from Pseudomonas putida GPo1 in both P putida GPp 104 and Escherichia coli JMU193. The polymerase encoding gene phaC1 was expressed using the inducible PalkB promoter. It was found that the production of polymerase could be modulated over a wide range of protein levels by varying inducer concentrations. The optimal inducer dicyclopropylketone concentrations for PHA production were at 0.03% (v/v) for P putida and 0.005% (v/v) for E. coli. Under these concentrations the maximal polymerase level synthesized in the E. coli host (6% of total protein) was about three-to fourfold less than that in R putida (20%), whereas the maximal level of PHA synthesized in the E. coli host (8% of total cell dry weight) was about fourfold less than that in P putida (30%). In P putida, the highest specific activity of polymerase was found in the mid-exponential growth phase with a maximum of 40 U/g polymerase, whereas in E. coli, the maximal specific polymerase activity was found in the early stationary growth phase (2 U/g polymerase). Our results suggest that optimal functioning of the PHA polymerase requires factors or a molecular environment that is available in R putida but not in E. coli.