Biological　foaming　is　a　major　problem　in　activated　sludge　(AS)　wastewater　treatment　systems.　In　this　study,　four　wastewater　treatment　plants　(WWTPs)　(a　total　of　six　AS　treatment　systems)　were　investigated.　The　microscopic　examination　shows　that　foaming　was　mainly　caused　by　gram-positive　short　branch　microorganisms,　sludge　fragments,　and/or　other　microorganisms,　while　the　long　unbranched　filamentous　was　easy　to　cause　bulking.　The　high　throughput　sequencing　(HTS)　and　Linear　discriminant　analysis　effect　Size　(LEfSe)　identified　the　significant　discrepancy　of　bacteria　in　foams　compared　to　normal　AS.　Mycobacterium,　Mycobacteriaceae,　Nocardiaceae,　Actinomycetales,　Chryseobacterium,　Flavobacterium,　Ormithobacterium,　Flavobacteriaceae,　and　Portibacter　were　considered　as　the　dominant　foaming-potential　bacteria　but　not　the　most　abundant　bacteria　in　the　foams.　The　excessive　growth　of　foaming　bacteria　(including　Haliscomentbacter,　Saprospiraceae,　and　Tetrasphra)　directly　led　to　bulking　with　a　high　sludge　volume　index　and　was　positively　correlated　with　sludge　retention　time　(SRT)　and　negatively　correlated　with　dissolved　oxygen　(DO),　which　means　long　SRT　and　low　DO　may　lead　bulking　instead　of　foaming.　It　also　found　that　the　foaming　bacteria　(including　Skermania,　Comamonadaceae,　Cloacibacterium,　Flavobacterium,　and　Chryseobacterium)　had　significant　positive　correlations　with　suspended　solids　and　mixed　liquid　suspended　solids,　and　negative　correlations　with　temperature　and　DO　concentration.　©　2025　by　the　authors.