Complementation analysis of a polyhydroxyalkanoate (PHA)-negative mutant of Aeromonas caviae proved that ORF3 in the pha locus (a 402-bp gene located downstream of the PHA synthase gene) participates in PHA biosynthesis on alkanoic acids, and the ORF3 gene is here referred to as phaJ(Ac). Escherichia coli BL21(DE3) carrying phaJ(Ac). under the control of the T7 promoter overexpressed enoyl coenzyme A (enoyl-CoA) hydratase, which was purified by one-step anion-exchange chromatography. The N-terminal amino acid sequence of the purified hydratase corresponded to the amino acid sequence deduced from the nucleotide sequence of phaJ(Ac) except for the initial Met residue. The enoyl-CoA hydratase encoded by phaJ(Ac) exhibited (R)-specific hydration activity toward trans-2-enoyl-CoA with four to six carbon atoms. These results have demonstrated that (R)-specific hydration of 2-enoyl-CoA catalyzed by the translated product of phaJ(Ac) is a channeling pathway for supplying (R)-3-hydroxyacyl-CoA monomer units from fatty acid beta-oxidation to poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) biosynthesis in A. caviae.