The production conditions of hydroxybutyrate monomers using Pseudomonas mendocina DS04-T-degradedpoly(3-hydroxybutyrate-co-4-hydroxybutyrate)[poly(3HB-co-4HB)] in submerged culture were optimized.The optimum culture medium (w/v)contained 0.15% poly(3HB-co-4HB), 0.1% (NH₄)₂SO₄, 1.6% Na₂HPO₄•12H₂O, 0.4% KH₂PO₄, 0.05% MgSO₄•7H₂O,and 0.0075% ZnSO₄•7H₂O. The optimum parameters for liquid fermentation were as follows: cultivation time, 28 h; temperature, 30 °C; rotary speed, 150 rpm; initial pH, 7.3; liquid-medium volume, 120 mL; and inoculum size, 1.5%.Under the optimum conditions, thehydroxybutyrate(HB) monomer yieldwas 43.2% ± 0.42%, the polyhydroxyalkanoatedepolymerase activity was39.6 ± 0.37U/mL, and theHB monomer yieldwas3.5times that under basic culture medium and initial conditions.