Abstract PURPOSE: Maternal colonization with GBS is a major cause of neonatal sepsis and meningitis. While GBS transmission is assumed to be fecal-oral or person-to-person direct contact including possible sexual transmission, risk factors for colonization and carriage are poorly described. Basic descriptive epidemiologic information is needed in order to implement a prevention program such as the development of an effective vaccine. METHODS: As part of a study of heterosexual transmission of urinary tract infection (UTI), we describe the prevalence and co-colonization of GBS among college couples. Self-collected vaginal, urine and rectal specimens were cultured from women with UTI and their current sex partners (“case couples”), and from women without UTI and their current sex partners (“control couples”). RESULTS: Thirty-four percent (98/285) of case women and 31% (56/182) of control women had at least one GBS isolate. 25% (45/177) of case men and 26% (32/122) of control men had at least one GBS isolate. Given that GBS was present in one partner, 45% of case (34/75) and 57% of control (24/42) couples were co-colonized. Using pulsed-field gel electrophoresis (PFGE) analysis, GBS isolates shared within couples were identical in most cases, but PFGE patterns differed across couples. CONCLUSIONS: Prevalence of GBS is high among sexually active college students and sexual contact is associated with increased transmission.