Chloroflexi are currently believed to serve as backbone forming agents in the activated sludge of wastewater treatment plants (WWTPs). In this study, we isolated and characterized filamentous bacteria in the class Caldilineae of the phylum Chloroflexi in municipal WWTPs. Diversity analysis using Chloroflexi-specific 16S rRNA gene clone libraries showed that 97% of the clones belonged to the subdivision Anaerolineae comprising the two classes Anaerolineae (95%) and Caldilineae (2%). Clones of Caldilineae were related to a thermophilic filament Caldilinea aerophila with 93% 16S rRNA gene sequence similarity. We obtained filamentous isolates classified into the class Caldilineae showing the best match to C. aerophila with 89% 16S rRNA gene sequence similarity. Isolates showed no ability to assimilate glucose or N-acetylglucosamine or to degrade biopolymers which were observed in filamentous Chloroflexi of WWTPs. The assessment of relative abundance based on quantitative PCR of the 16S rRNA gene indicated that members of the class Caldilineae comprised 12-19% of the Chloroflexi in the activated sludge. Additionally, fluorescence in situ hybridization experiments showed that diverse filamentous Caldilineae inhabit the activated sludge of municipal WWTPs. These findings yield insight into the role of filamentous mesophilic Caldilinea in stabilizing flocs of activated sludge in a wide range of WWTPs.