Trichomonas vaginalis is a sexually transmitted anaerobic parasite which causes vaginitis and cervicitis. The present study was carried out to determine the incidence of Trichomonas 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 x 3x a day for 10 days). The semen obtained from 1131 men was examined for different characteristics and Trichomonas 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 less than 0.001). There was no significant change in the pH of semen. Spermatozoan motility and morphologically normal forms were decreased significantly (P less than 0.001), spermatozoan viability was altered, and there was a significant change in membrane integrity (P less than 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 by Trichomonas.