The investigation indicated the presence and the properties of cytidine deaminase(EC.3.5.4.5) activity in extract of the parasite Trichomonas vaginalis grown in Diamond's TYM medium. The optimum conditions for the enzyme activity was determined and enzyme activity was obtained in a reaction mixture of (400)µM of Tris-HCl buffer at pH (7.2) containing (7) mM of cytidine as a substrate and an amount of the enzyme ranging from (200-250)µg of the enzyme extract, and an incubation period of (10) min. at (37)°C. Under the optimum conditions, the specific activity was found to be (17.5) mM of cytidine deaminated per min/ mg protein in the supernatant of Trichomonas vaginalis extracts. the investigated Results of the properties of the cytidine deaminase activity showed that cytidine monophosphatet(CMP)might function as alternative substrate for enzyme activity.