Metronidazole ester, a new nitroimidazole drug, was synthesized by the Department of Physical Chemistry, School of Pharmacy, Shanghai Medical University. The trichomonadicidal and amoebicidal activity of metronidazole ester was evaluated in vitro by the cultivation of Trichomonas vaginalis and Entamoeba histolytica at different drug concentrations. The results indicated that the 100% lethal concentration for T. vaginalis and E. histolytica was 2.6 micrograms/ml and 40 micrograms/ml, respectively after incubation with metronidazole ester for 48 h. In comparison with metronidazole, the effect of metronidazole ester for both protozoa was similar.