Strains of Trichomonas vaginalis were evaluated for their in vitro responses to metronidazole. A serial twofold dilution method that measured minimal inhibitory concentrations (MICs) by cessation of trophozoite motility was compared with the disk broth method, in which both cessation of motility and inhibition of trophozoite proliferation were used for measurement of MICs. These methods gave comparable results in distinguishing a metronidazole-resistant and two metronidazole-susceptible strains of T. vaginalis under conditions of both aerobic and anaerobic incubation. However, anaerobic test conditions significantly lowered the MIC of metronidazole against a metronidazole-resistant strain. Clinical isolates of T. vaginalis and strain ATCC 30001 from the American Type Culture Collection (Rockville, Md.) were also tested. Two of 12 clinical isolates were resistant to metronidazole in vitro. It is concluded that the disk broth method is a simple and reliable screening method for detection of metronidazole-resistant strains of T. vaginalis.