Chironomid　larvae　(CL),　aquatic　invertebrates　in　freshwater　environments　are　the　common　macro-scale　organisms　in　activated　sludge　wastewater　treatment　systems.　The　effects　of　CL　on　biomass　quantity,　sludge　characteristics,　and　nitrogen　removal　efficiency　were　investigated　in　a　domestic　wastewater　treatment　system　in　this　study.　The　batch　tests　showed　that　CL　induced　sludge　reduction　and　enhanced　sludge　settlement.　A　maximum　sludge　weight　reduction　rate　of　42%　was　achieved　at　the　CL　wet-weight　density　of　0.5　g　CL/g　total　suspend　solid　(TSS),　and　the　sludge　volume　index　(SVI)　decreased　from　146　to　78　mL/g　as　the　CL　wet-weight　density　increased　from　0　to　0.5　g　CL/g　TSS.　After　52　operation　cycles,　the　system　with　the　CL　density　of　0.5　g　CL/g　TSS　had　a　lower　sludge　yield　(0.22　g　volatile　suspended　solid　(VSS)/g　removed　chemical　oxygen　demand　(CODremoved)　than　the　control　reactor　without　CL　addition　(0.34　g　VSS/g　CODremoved).　Addition　of　CL　enhanced　the　simultaneous　nitrification　and　denitrification　(SND),　but　partially　inhibited　nitrification.