Incidence of Herpes simplex virus (HSV) was studied in ejaculates of 100 men. The examinees had neither history nor clinical symptoms of HSV genital infection. HSV was detected by a rapid cultural method in the ejaculate of 20 out of 100 examinees (20%). Of 67 males with infertility HSV was detected in 25%, in 19 males examined prophylactically and 14 patients with varicocele (a comparison group) it was found in 10.5 and 7% cases, respectively. Oligozoospermia was two times more frequent in HSV-containing ejaculates than in HSV-negative one. Mean values of the majority of sperm parameters in HSV-positive and HSV-negative groups did not differ statistically. However, it was revealed that such damage of the spermatozoa structure as microhead (consequence of the defective acrosome or reduced genome) and cytoplasm drops on the neck (a sign of immature forms) occurred more often in HSV-infected patients than in persons with HSV-negative ejaculate. Thus, asymptomatic HSV infection has a negative effect on male fertility.