Kouleothrix (Type 1851) filamentous bacteria associated with bulking incidents were investigated for their quantitative correlations with sludge settleability in municipal wastewater treatment plants (WWTPs) around Japan. Factors affecting their abundance were also explored by statistical analyses with the aim of controlling bulking by suppressing their growth. Real-time quantitative PCR assays showed Kouleothrix spp. mainly ranged from 10(4) -10(5 )copies ng-DNA(-1) in WWTP5. They were not always abundant in every wwrp, but their abundance (>10(5) copies ng-DNA(-1)) almost always coincided with low activated sludge settleability. Their abundance in nitrogen removal processes was always low (1.08 - 3.83 x 10(4) copies ng-DNA(-1)). Denitrification process at the feeding stage may be a useful selection pressure for Kouleothrix spp., leading to their suppression. A stepwise multiple regression analysis revealed that the increase of dissolved oxygen (DO) and pH in an aeration tank have a negative influence, while NH4+ increase in the influent has a positive influence on the Kouleothrix spp. abundance. The three-variable model was significant and accounted for approximately 45% of the variance in the dependent variable (i.e., Kouleothrix spp.). The DO increase had the greatest influence on the Kouleothrix spp. decrease, and thus, the DO control may be a promising way to suppress their growth in WWTPs. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.