A Gram-stain-negative, heterotrophic, aerobic, non-spore-forming and non-motile bacterial strain, designated LM5(T), was isolated from activated sludge from a sequencing batch reactor for the treatment of effluents contaminated by malachite green. The taxonomy of strain LM5(T) was studied by phenotypic and phylogenetic methods. Strain LM5(T) formed orange colonies on R2A and YP plates. Cells were rods, 0.4-0.6 microm in diameter and 0.8-1.2 microm in length. Growth occurred at 10-35 degrees C (optimum, 20-25 degrees C), at pH 5.5-9.5 (optimum, pH 6.5-7.5) and in the presence of 0-2 % (w/v) NaCl (optimum, 0.5 %). Oxidase and catalase activities were present. Flexirubin-type pigments were present, but extracellular glycans were absent. MK-6 was the major respiratory quinone. The major fatty acids were iso-C(15 : 0) (28.3 %) and iso-C(17 : 1)omega9c (13.8 %). Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain LM5(T) was a member of the genus Flavobacterium with highest sequence similarity to Flavobacterium soli DS-6(T) (93.2 %) and Flavobacterium lindanitolerans IP-10(T) (92.9 %). Together with F. lindanitolerans IP-10(T), strain LM5(T) formed a distinct lineage in the phylogenetic tree. The DNA G+C content was 52+/-0.6 mol% (HPLC), which is significantly higher than that of other species of the genus Flavobacterium (30-41 mol%). Based on phylogenetic and phenotypic evidence, strain LM5(T) is considered to represent a novel species of the genus Flavobacterium, for which the name Flavobacterium caeni sp. nov. is proposed; the type strain is LM5(T) (=CGMCC 1.7031(T)=NBRC 104239(T)).