Trichomonas vaginalis is a sexually transmitted anaerobic parasite which causes vaginitis and cervicitis. The present study was carried out to determine the incidence ofTrichomonas infection in semen samples of asymptomatic men and also to understand the changes of semen characteristics in the infected individuals before treatment and after treatment with metronidazole (Flagyl, 400 mg×3× a day for 10 days). The semen obtained from 1131 men was examined for different characteristics andTrichomonas were detected in wet smear preparations in 50 cases (4.42%). The characteristics of semen in them was compared with 52 normal semen samples. Statistical analysis showed that the seminal fluid viscosity and percentage particulate debris was increased significantly in the infected group (P<0.001). there="" was="" no="" significant="" change="" in="" the="" ph="" of="" semen.="" spermatozoan="" motility="" and="" morphologically="" normal="" forms="" were="" decreased="" significantly="">P<0.001), spermatozoan="" viability="" was="" altered,="" and="" there="" was="" a="" significant="" change="" in="" membrane="" integrity="">P<0.001) as="" indicated="" by="" the="" hypoosmotic="" swelling="" test.="" the="" significant="" improvement="" in="" semen="" characteristics="" were="" seen="" in="" 25="" cases="" after="" a="" single="" course="" of="" treatment.="" therefore,="" it="" is="" possible="" that="" in="" some="" cases,="" the="" infertility="" seen="" in="" asymptomatic="" individuals="" may="" be="" due="" to="" infection="">Trichomonas.