Analysis of native poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] inclusions from Aeromonas caviae FA440 revealed that ORF1 (a 348-bp gene located immediately upstream of phaC(Ac)) encodes a 13-kDa granule-associated protein, which was referred to as phaP(Ac). Several recombinant strains of A. caviae were constructed and conducted to analyze their PHA-producing abilities. A transconjugant of FA440 harboring additional copies of phaPCJ(Ac) genes accumulated P(3HB-co-3HHx) copolyesters with much higher 3HHx composition (46-63 mol %) than wild-type strain from alkanoates or olive oil. Deletion analysis revealed that overexpression of phaJ(Ac) encoding monomer-supplying (R)-hydratase was not a reason for the compositional change in the recombinant strains. PHA synthase activity in PHA inclusion fraction from the transconjugant composed of 60 mol % of 3HHx was 10-fold higher than that from the strain FA440 with 13 mol % of 3HHx, suggesting an importance of the level of PHA synthase activity for controlling the PHA composition in vivo.