Group B streptococci are an important cause of infant septicemia and meningitis. A prospective study of group B streptococcal colonization in a 300-bed community hospital disclosed rates of 29% of 297 third-trimester women, 37% of 242 newborn infants, and 45% of 22 hospital personnel. Colonized parturients were more frequently black and anemic on admission for delivery. Infant colonization was statistically associated with a positive maternal genital culture, low birthweight, and prematurity. Nosocomial transmission of group B streptococci was strongly suggested by observations that 41% of colonized infants were born to culture-negative women and such infants became colonized later in their hospital stay than did colonized infants born to colonized women. Furthermore, hospital personnel working in the labor-delivery and nursery areas had a significantly higher prevalence of the organism than did personnel from other areas. Clearly, more information is needed about the epidemiology of group B streptococcal disease before appropriate and rational control measures can be recommended.