Strain NA10B^T and other two strains of the denitrifying betaproteobacterium Diaphorobacter nitroreducens were studied for the performance of solid-phase denitrification (SPD) using poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and some other biodegradable plastics as the source of reducing power in wastewater treatment. Sequencing-batch SPD reactors with these organisms and PHBV granules or flakes as the substrate exhibited good nitrate removal performance. Vial tests using cultures from these parent reactors showed higher nitrate removal rates with PHBV granules (ca. 20 mg-${\mathrm{NO}}_3^{-}‐\mathrm{N}$ g⁻¹ [dry wt cells] h⁻¹) than with PHBV pellets and flakes. In continuous-flow SPD reactors using strain NA10B^T and PHBV flakes, nitrate was not detected even at a loading rate of 21 mg-${\mathrm{NO}}_3^{-}‐\mathrm{N}$ L⁻¹ h⁻¹. This corresponded to a nitrate removal rate of 47 mg-${\mathrm{NO}}_3^{-}‐\mathrm{N}$ g⁻¹ (dry wt cells) h⁻¹. In the continuous-flow reactor, the transcription level of the phaZ gene, coding for PHB depolymerase, decreased with time, while that of the nosZ gene, involved in denitrificaiton, was relatively constant. These results suggest that the bioavailability of soluble metabolites as electron donor and carbon sources increases with time in the continuous-flow SPD process, thereby having much higher nitrate removal rates than the process with fresh PHBV as the substrate.