Changes in bacterial communities in the foam,corresponding foaming AS,and non-foaming AS from a seasonal foaming wastewater treatment plant(WWTP)in northern China were investigated by high-throughput pyrosequencing and molecular quantification-based approaches.We found that bacterial communities of the foam and the corresponding foaming AS were similar but markedly different from those of the non-foaming AS.Actinobacteria was the predominant phylum in the foam and the corresponding foaming AS,while Proteobacteria was predominant in the non-foaming AS.Similar to most previous studies,Candidatus Microthrix parvicella was the predominant filamentous bacteria in the foam and the corresponding foaming AS,and this specieswas significantlyenriched in the foam compared to corresponding foaming AS.Its abundance decreased gradually with a slow disappearance of sludge foaming,indicating its overgrowth had a direct relationship with sludge foaming.In addition to CandidatusMicrothrix parvicella,Tetrasphaera and Trichococcusmight play a role in sludge foaming,becausethey supported the changes of ASmicrobial ecology for foam formation.Effluent water quality of this surveyed plant remained stable during the period of sludge foaming,but the microbial consortia responsible for nitrogen and phosphorusremoval markedly changed compared to that in the non-foaming AS.This study adds to the previous understanding of bacterial communities causing foaming in WWTPs.