A novel Bacteroides fragilis selective (BFS) medium, consisting of a brain heart infusion agar base supplemented with yeast extract, cysteine hydrochloride, bile salts, vitamin K, hemin, glucose, esculin, ferric ammonium citrate, bromothymol blue, gentamicin, kanamycin, and novobiocin, was evaluated. When BFS agar was tested with a collection of 303 bacteria of different genera, it allowed the growth of B. fragilis as large yellow colonies, with blackening of the medium after 48 h of anaerobic incubation, while the growth of most other anaerobes, facultative anaerobes, and aerobes was inhibited. In a prospective comparison of BFS agar with a routinely used medium (neomycin blood agar) in 1,209 clinical specimens, 60 B. fragilis bacteria were detected on BFS agar while 46 were detected on the routine agar (McNemar's test, P ϭ 0.008). In conclusion, this novel medium may be added to improve the recovery of B. fragilis in clinical specimens and to facilitate surveillance of antimicrobial-resistant strains. KEYWORDS selective medium, Bacteroides, media B acteroides species colonize the human gut as commensals, but they can also cause infections if translocated into the bloodstream or other tissues following disease, an operation, or trauma (1, 2). In the genus Bacteroides, Bacteroides fragilis is regarded as the most virulent species and has been implicated in various infections such as intra-abdominal sepsis, deep-seated abscesses, and necrotizing skin and soft tissue infections (2). In B. fragilis, emerging resistance to antibiotics is a big concern (3, 4). Among isolates from large hospitals in the United States and Europe, resistance rates for clindamycin and moxifloxacin are now Ͼ20% to 30% (3, 4). Currently, carbapenem and metronidazole resistance rates among B. fragilis isolates from the United States and Europe remain low (Ͻ1% to 2%) in general (3, 4). However, higher carbapenem resistance rates have been reported by investigators from Taiwan (7% to 12%) (5), Germany (4.9% to 5.3%) (1), and Canada (2.3% to 12.7%) (6), suggesting that this resistance phenotype may be emerging in certain hotspots in those countries. Most seriously, rare strains of multidrug-resistant B. fragilis with simultaneous resistance to carbapenems and metronidazole have also been identified (7, 8). In B. fragilis, two divisions (I and II) have been identified by using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) (9). Metronidazole is administered as an inactive prodrug, and reduction of the nitro group is required for activation to the active compounds. In B. fragilis, metronidazole resistance is usually attributed to acquisition of the nitroimidazole resistance gene (nim), encoding a nitroimidazole reductase which is presumed to modify metronidazole to an inactive compound (10). The acquisition of carbapenem resistance usually correlates with the activation of the carbapenemase gene, cfiA, in division II strains by an upstream insertion sequence (11). Most of the genetically characterized ...