A study of the faecal microbiome in three healthy female rhesus macaques revealed the presence of a novel obligately anaerobic, chemoorganoheterotrophic, non-sporing, coccoid, non-motile, Gram-stain-positive bacterial species. Three strains of this species, designated as M108T, M916-1/1, and M919-2/1, were non-haemolytic, H2S-positive, catalase-positive, bile- and NaCl-sensitive and required peptone for growth. Strains also were asaccharolytic, able to utilize sulfite, thiosulfate and elemental sulfur as electron acceptors, and produced acetic and butyric acids as metabolic end-products. Strain M108T is characterized by the prevalence of C14 : 0, C16 : 0 and C18 : 1ω9cis dimethyl acetal among the cellular fatty acids, and the presence of MK-10 menaquinone. The DNA G+C content was found to be 51 mol%. Phylogenetic analysis of partial 16S rRNA gene sequences of strains M108T, M916-1/1 and M919-2/1 placed these strains into the genus Peptococcus (family Peptococcaceae). On the basis of phenotypic and genotypic properties we conclude that these strains represent a novel bacterial species for which the name Peptococcus simiae sp. nov. is proposed. The type strain is M108T (=DSM 100347T=VKM B-2932T).