A Gram-negative bacterium, designated P2(T), was isolated from the biofilm developed on the inner surface of an ultrapure cooling water system in a Hungarian power plant and was characterized by a polyphasic approach. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain P2(T) was affiliated with the family 'Flexibacteraceae' in the phylum Bacteroidetes. Its closest relative was Flectobacillus lacus CL-GP79(T) (88.7 % 16S rRNA gene sequence similarity) followed by Arcicella rosea TW5(T) (86.5 %), Arcicella aquatica NO-502(T) (86.4 %), Flectobacillus roseus GFA-11(T) (86.3 %) and Flectobacillus major DSM 103(T) (85.4 %). Cells of strain P2(T) were facultatively anaerobic, non-motile rods. The major fatty acids were C(16 : 1)ω5c (42.5 %), iso-C(15 : 0) 2-OH (17.2 %), iso-C(17 : 0) 3-OH (16.1 %) and iso-C(15 : 0) (8.5 %). The major menaquinone was MK-7 and the predominant polar lipid was phosphatidylethanolamine. The DNA G+C content was 54.5 mol%. Thus, the phenotypic and genotypic analyses clearly showed that strain P2(T) is considerably different from members of other genera in the family 'Flexibacteraceae'. Based on these results, it is concluded that strain P2(T) represents a novel species in a new genus, for which the name Siphonobacter aquaeclarae gen. nov., sp. nov. is proposed, with type strain P2(T) (=DSM 21668(T) =NCAIM B 02328(T)).