High rates of carriage of group B streptococci were found among men (38%) and women (42 3 %) attending a clinic for sexually transmitted diseases. Swabs from the perineal/ anorectal area gave the highest isolation rate and those from the urethra the lowest. The subpreputial sac was an important site for carriage of the organism, and there was a strong association between streptococcal isolation and balanitis. Of 92 couples studied, neither partner was colonised with group B streptococci in 36. In a further 36 one or other was colonised and in 20 both were colonised. Serotyping and phage typing showed that only three of these 20 couples were colonised with similar strains of the organism.