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Bacteroides microfusus sp. nov. is described on the basis of 16 strains isolated from a number of fecal or cecal specimens from calves, chickens, and Japanese quails. The isolates are obligately anaerobic, gram-negative, nonsporeforming, nonmotile, relatively small rods with pointed ends; the cells occur singly, in pairs, and sometimes in short chains. B. microfusus differs from other species in the genus Bacteroides principally in morphology; the small spindle-shaped cells, particularly on agar media, are rather distinctive. The guanine-plus-cytosine contents of the deoxyribonucleic acids of the new strains range from 59 to 61 mol%, the highest value so far detected in the genus Bacteroides. The new strains are similar to those of B. putredinis, B. furcosus, B. amylophilus, and B. succinogenes in fermenting a relatively small number of carbohydrates; however, B. microfusus can clearly be differentiated from these organisms by numerous biochemical properties and/or fermentation products. The type strain of B. microfusus is Q-1 (= ATCC 29728 = NCTC 11190).