An investigation to determine the throat and vaginal colonization rates of group B streptococci among second- and third-trimester pregnant women was performed. Group B streptococci were recovered from 25.4 per cent of third-trimester and 14.8 per cent of second-trimester parturient (p = less than 0.025). Serotype distribution of isolates was similar among these two study groups. No significant differences in colonization rates were noted on the basis of age, race, parity, or complications of pregnancy.However, postpartum fever occurred in 22 per cent of colonized and only 4 per cent of noncolonized second-trimester parturients. The reason for this significant increase in group B streptococcal colonization rates with advancing gestation remains speculative.