An animal model for group B streptococcal vaginal colonization and neonatal acquisition was developed with albino rats. Intravaginal inoculation of genital isolates of group B streptococci of serotypes Ia, II, and III either once or on 3 successive days resulted in carriage of the organisms for 7 days or longer in 26% of the virgin animals and 43% of the pregnant animals. Throat and perianal cultures of the offspring of pregnant rats revealed that 51% of the rat pups acquired the organisms at some time. Litter exchange studies were done to explore the contributions of environmental and intralitter spread. Significantly more infants born to mothers with positive vaginal cultures acquired the organisms than infants of culture-negative mothers who were suckled by positive adoptive mothers. However, 13% of the offsprinital cultures acquired group B streptococci. This model may be valuable in understanding the dynamics of vaginal carriage and mother-infant transmission of group B streptococci.