Summary: Three antimicrobial nitroimidazole drugs (metronidazole, dimetridazole, and tinidazole) inhibit a range of Clostridia and the protozoan Trichomonas vaginalis; they have an identical site and mode of action as specific electron acceptors from the pyruvate phosphoroclastic reaction. Analogues of the drugs are compared and the structural requirements for activity explained. The nitrofuran (nitrofura-zone) probably has a different mechanism of action.