The　starting　conditions　of　filamentous　bulking　and　the　transition　to　adverse　bulking　in　sewage　treatment　were　investigated　in　an　anoxic/oxic　(A/O)　process　under　low　dissolved　oxygen　(DO)　with　domestic　sewage　of　low　carbon/nitrogen　ratio.　Sludge　volume　index　(SVI)　values　increased　and　maintained　under　250　mug　when　the　DO　value　was　0.5　mg/L,　basically　in　line　with　the　characteristics　of　limited　filamentous　bulking　under　low　DO.　In　this　period,　the　average　removal　rates　of　COD　and　total　nitrogen　(TN)　were　improved　by　2.23%　and　7.75%,　respectively,　compared　to　the　high　DO　stage.　Haliscomenobacter　hydrossis　were　the　dominant　filamentous　bacterium　in　bulking　sludge.　The　network　structure　formed　by　filamentous　bacteria　played　a　filtering　role　such　that　the　concentration　of　suspended　solids　(SS)　in　the　effluent　was　almost　0.　However,　the　SW　climbed　to　nearly　500　ml/g　after　70　days　and　adverse　bulking　occurred.　Compared　to　the　early　stage,　the　removal　rates　of　NH4+-N,　COD,　and　TN　decreased　in　the　late　stage.　Fluorescent　in　situ　hybridization　(FISH)　showed　that　the　main　filamentous　bacteria　leading　to　bulking　were　also　H　.hydrossis.