The distribution of metronidazole susceptibility factors in clinically important bacteria was studied to determine why obligate anaerobes were susceptible to metronidazole and facultative ones were not. With obligate and facultative anaerobes, nitroreductase activity, metronidazole uptake, pyruvate:ferredoxin oxidoreductase activity and drug susceptibility were estimated. Nitroreductase activity was distributed in the obligate anaerobes and some facultative ones such as Streptococcus faecalis. Propionibacterium acnes, Bifidobacterium adolescentis, Actinomyces israelii and most of the facultative anaerobes had no such activity. Bacteria with nitroreductase activity were able to absorb the drug. Pyruvate:ferredoxin oxidoreductase activity was distributed in all of the obligate anaerobes, whereas Prop. acnes, Bif. adolescentis, A. israelii and all of the facultative anaerobes had no such activity. Distribution of metronidazole susceptibility was the same as that of pyruvate:ferredoxin oxidoreductase activity. Therefore metronidazole may selectively inhibit obligate anaerobes with pyruvate:ferredoxin oxidoreductase activity. Nitroreductase activity may be necessary for drug uptake as well.