Filamentous　bulking　is　a　complicated　problem　in　wastewater　treatment　plants　treating　various　wastewaters,　leading　to　the　deterioration　of　the　settling　properties　and　the　effluent　quality.　This　study　systematically　investigated　long-term　effects　of　various　carbon　sources　and　feeding　patterns　on　the　growth　of　filamentous　bacteria,　in　order　to　reveal　the　mechanism　of　filamentous　bulking.　Sludge　volume　index　(SVI),　microscopic　observations,　staining　(Gram　and　Neisser　staining),　scan　electron　microscopic,　and　fluorescent　in　situ　hybridization　(FISH)　were　used　to　monitor　the　bulking　and　track　the　changes　of　microbial　morphology　and　community　structure　of　activated　sludge　in　six　lab-scale　sequencing　batch　reactors　(SBRs)　fed　with　different　carbon　sources.　Filamentous　bulking　was　not　observed　in　all　SBRs　under　anoxic　feeding　pattern　with　a　short　fill　time,　in　which　SVI　remained　below　150　mL/g.　In　contrast,　serious　bulking　(SVI　＞　500　mL/g)　occurred　under　aerobic　feeding　pattern　when　fed　with　ethanol,　propionate,　acetate,　and　glucose,　in　which　Thiothrix　and　Sphaerotilus　natans　proliferated　as　dominant　filaments.　Compared　to　glucose-fed　reactor,　relatively　light　bulking　was　caused　in　starch-fed　reactor　with　the　growth　of　Nostocoida　limicola　II.　In　addition,　flocs　in　starch-fed　reactor　were　more　open　and　fluffy　than　flocs　formed　on　readily　biodegradable　substrates.　Finally,　a　framework　integrating　kinetic　selection,　diffusion　selection,　storage　selection,　and　protozoa　capture　mechanism　was　proposed　to　explain　filamentous　bulking.