Leptothrix cholodnii is a sheathed bacterium often found in metal-rich and oligotrophic aquatic environments. A bacterial strain that is able to degrade the NaOH-treated sheath of L. cholodnii was isolated. The isolate was a Gram-negative, aerobic and prosthecate bacterium. The optimum growth temperature and pH were 30 degrees C and pH 7.0, respectively. The DNA G+C content was 62.9 mol%. The major respiratory quinone was MK-6. A phylogenetic analysis based on the 16S rRNA gene indicated that the isolate is a member of the genus Prosthecobacter. The nearest relative was the type strain of Prosthecobacter vanneervenii, with a similarity of 97.1 %. However, the isolate does not possess the bacterial tubulin genes, btubA and btubB, unique to known species of the genus Prosthecobacter. It is proposed that the isolate represents a novel species, Prosthecobacter fluviatilis sp. nov. The type strain is HAQ-1(T) (=JCM 14805(T) =KACC 12649(T) =KCTC 22182(T)).