Trichomonas vaginalis vaginitis is generally treated with oral metronidazole. Widespread use of this drug has led, however, to selection of resistant strains. Topical therapy seems appropriate whenever systemic high dosage schedule cannot be adopted in order to overcome resistance. This study was designed to assess the activity of metronidazole alone and in combination with clotrimazole, an imidazole derivative for topical use, against T. vaginalis. Tests were performed employing the antitrichomonas activity of a fixed ratio of metronidazole with clotrimazole (5:1) which has been recently suggested for topical therapy and the checkerboard technique. All tests were carried out under aerobic conditions to maximize T. vaginalis resistance traits. Minimum inhibitory concentrations (MICs) of metronidazole for the 12 strains studied were in the range reported in the literature (0.5-32 micrograms/ml). The interaction of metronidazole with clotrimazole as assessed by the checkerboard technique gave an indifferent outcome with all the strains assayed (FIC = 1-2). The fixed concentration of drugs, however, produced synergism (FIC = 0.5) in 5 of 12 isolates. Spontaneous resistant strains were not selected from T. vaginalis exposed to sub-lethal levels of the drugs or by culturing a large inoculum in the presence of 1, 2, 4 and 8 times the MICs of metronidazole alone or in combination with clotrimazole. These results confirm and extend previous reports highlighting the good in vitro efficacy of the association of metronidazole plus clotrimazole against T. vaginalis.