A longitudinal study of colonization by group B streptococcus (GBS) was conducted in 93 pregnant women and 92 of their infants. Positive cultures were obtained from vaginal swab specimens on at least one occasion during pregnancy from 20.4% of the women. Three types of carriage were observed: chronic, transient, and intermittent. At the time of delivery, 12.9% of the mothers were colonized by GBS. GBS colonization occurred in 41.7% of the infants of the mothers colonized intrapartum. Only 1.2% of infants intrapartum culture-negative mothers were colonized. Infants colonized at birth were culture negative by 4 months of age. This study suggests that interruption of vertical transmission of GBS from mother to infant not only would influence the incidence of early-onset GBS disease but also might reduce the incidence of late-onset GBS disease.