The in vitro activities of tinidazole and metronidazole against Trichomonas vaginalis isolates clinically resistant to metronidazole were compared. Minimal lethal concentrations (MLCs) of tinidazole were significantly lower than MLCs of metronidazole. Increased metronidazole resistance correlated with increased tinidazole resistance. These data support a role for tinidazole in the treatment of trichomoniasis.