Carboxyphilic strains which had characteristics commonly attributed to the group D streptococci and especially to Streptococcus bovis were isolated from the ceca of chickens. However, these strains differed from S. bovis in being positive for ribose, melezitose, P-glucuronidase, and alkaline phosphatase, and negative for group D reactions and growth enhancement by C02. These differences were confirmed by deoxyribonucleic acid-deoxyribonucleic acid hybridization tests. The strains form a new species, for which the name Streptococcus c'ecorum is proposed. Strain A60 (=NCDO 2674) is the type strain of this species.During an investigation of the susceptibilities of intestinal bacteria isolated from chickens to antimicrobial agents used in farm animals (G. N . Dutta, D. Vet. Sci. thesis, University of Gent, Gent, Belgium, 1983), a group of carboxyphilic streptococci was found which could not be identified as any known streptococcal species. The strains in this group were able to grow at 45°C and in the presence of 40% bile, two characteristics which are commonly attributed only to the enterococci or group D streptococci. After the tests advocated by Facklam (6) and Sharpe (21) for the separation of fecal streptococci from other streptococci (growth at pH 9.6, in 6.5% NaC1, and at 10 and 45°C) were performed, the strains were tentatively identified as Streptococcus bovis. However, three reactions were in discordance with this identification; the strains failed to react with the Streptococcus group D antisera used, they were able to grow to a limited extent at pH 9.6, and their growth was definitely better in an atmosphere containing 5% C02 than in air.A subsequent biochemical and genetic study showed that these strains should be designated a new species, which is described here. MATERIALS AND METHODSStrains. Strains A2a, A16, A29, A37 through A40, and A60T (T = type strain) were isolated from the ceca of eight chickens from eight farms in Belgium by using Islam medium (12) supplemented with 30 Fg of neomycin per ml and 15 pg of nalidixic acid per ml. The plates were incubated for 48 h in 5% COz in air. S . bovis CCM 5616 was obtained from the Czechoslovak Collection of Microorganisms, Brno. Other reference strains were obtained from the National Collection of Dairy Organisms (NCDO), Shinfield, Reading, England. _____Media and culture incubation. Media were obtained from Oxoid Ltd., Basingstoke, England, unless otherwise stated. Cultures were normally incubated at 37°C.Growth characteristics. Growth on sheep bloodColumbia agar plates was compared after incubation aerobically and in a 5% C 0 2 atmosphere for 16 and 48 h. All of the other tests except the API tests were performed in a 5 to 10% C 0 2 atmosphere.Biochemical reactions. Dextran and levan formation was tested on heart infusion agar supplemented with 5% (wthol) sucrose. Acid production was tested in phenol red broth base (GIBCO, Paisley, Scotland) containing 1% (wthol) carbohydrate. Glycerol fermentation was tested anaerobically on the solidified medium descri...