Eight anaerobic, pigmented, non-spore-forming, Gram-negative, rod-shaped strains isolated from monkey oral cavities were characterized phenotypically and chemotaxonomically and their phylogenetic positions were determined using 16S rRNA gene sequence analysis. The 16S rRNA gene sequence analysis showed that these isolates represent a single species of the genus Prevotella. These strains were most closely related to Prevotella intermedia ATCC 25611(T), with 95.0 % 16S rRNA gene sequence similarity. The next most closely related species were Prevotella pallens and Prevotella nigrescens (92.7 and 92.1 % similarity to the respective type strains). The phenotypic and biochemical characteristics of the isolates were the same as those of P. intermedia JCM 12248(T) and P. nigrescens JCM 12250(T). The isolates could be differentiated from P. pallens JCM 11140(T) on the basis of mannose fermentation and alpha-fucosidase activity. The isolates could not be distinguished from P. intermedia or P. nigrescens using conventional biochemical tests. DNA-DNA hybridization experiments revealed the genomic distinctiveness of these eight strains with respect to P. pallens JCM 11140(T), P. intermedia JCM 12248(T) and P. nigrescens JCM 12250(T). On the basis of these data, strains 04013, 04021, 04043, 04052(T), 0406, 04113, 04111 and 04161 represent a novel Prevotella species, for which the name Prevotella falsenii sp. nov. is proposed. The type strain is 04052(T) (=JCM 15124(T) =CCUG 56137(T)).