Activated-sludge　bulking　was　found　in　a　pilot-scale　SBR　with　long-term　operation　over　200　days,　fed　with　real　domestic　wastewater.　This　paper　focuses　on　the　analysis　of　the　relationship　between　sludge　bulking　and　factors　such　as　temperature,　the　content　of　extracellular　polymeric　substances　(EPS),　sludge　loading　and　DO.　The　results　showed　that　non-filamentous　activated-sludge　bulking　generally　occurred　at　low　temperature　(16°C),　which　led　to　poor　settleability　of　the　sludge.　The　MLSS　of　the　SBR　gradually　decreased　to　less　than　1200　mg·L-1　when　non-filamentous　activated-sludge　bulking　was　found　in　the　system.　The　content　of　EPS　in　the　bulking　sludge　was　higher　than　that　in　the　normal　sludge,　and　bulking　sludge　was　difficult　to　recover　although　DO　and　temperature　were　2.0　mg·L-1　and　20°C,　respectively.　Finally,　the　reasons　causing　activated-sludge　bulking　were　analyzed　and　preventive　approaches　are　suggested.