Vaginal, cervical, and rectal swabs were obtained from 762 women early in pregnancy to culture group B streptococci (GBS). The overall carrier rate was 13.9% and the rectal, vaginal, and cervical carrier rates were 10.6%, 7.9%, and 6.3%, respectively, GBS were isolated only from the rectum in 33%, so the rectum is an important reservoir. Seventy-five percent of the culture-positive women were permanent or intermittent carriers. Ninety percent of all women positive at labor were positive before the 20th week; acquisition of GBS late in pregnancy was rare. The frequency of transmission to the neonates was 63.4% among permanent carriers. Other determinants for transmission were the number of positive sites, the heaviness of colonization, and the (cervical) localization of GBS at labor. The serotypes of GBS isolated from the mothers and neonates were the same. Serotype distribution fluctuated during pregnancy. Types III (29%) and Ib (27%) were predominant, followed by types II (12%) and Ic (10%).