The relationship of sexual activity to vaginal colonization with the genital mycoplasmas (T-strain mycoplasmas andMycoplasmahominis) was studied in 183 student and graduate nurses. Participants with no history of sexual contact were virtually free of mycoplasmas, whereas 37.5% of those with a history of intercourse with a single partner and 75.0% of those who had had intercourse with three or more partners were colonized with T-mycoplasmas. These data suggest that T-mycoplasmas are acquired through sexual contact and are part of the vaginal flora of healthy sexually active young women.