We retrospectively analyzed the colonization rates for GBS in cultures obtained at delivery in two groups of women: Group One, low socioeconomic level, and Group Two, made up of middle and upper middle class women. Cultures were obtained immediately before vaginal or operative delivery sing single lower vaginal swab and anorectal swabs. All deliveries taking place between 1990 and 1994 were studied. The swabs were placed in transport media, and transferred to solid media within 12 hours. Serotyping with specific antisera was done whenever possible. Urethral and anorectal cultures from six partners were obtained if the partner consented. The number of live births for Group One/Group Two for each consecutive year studied, were respectively: 5,617/243; 5,512/251; 6,114/283; 5,930/250, and 5,016/249. Positive cultures for Group One women for each year reached: 18 (0.32%); 22 (0.39%); 89 (1.45%); 114(1.93%) and 212 (4.22%); combined total: 0.17% (455/28,189). Among Group Two women the figures were 6 (2.48%); 8 (3.51%); 8 (2.82%); 7 (2.81%), and 15(6.02%); combined total: 3.44% (44/1,276) (p=0.0001; odds ratio 0.47; CI 0.43-(4.08%) had positive urethral cultures and 32 (2.66%) had positive anorectal samples. Another 16 (1.33%) showed positive in both cultures. Combined incidence was 8.16% (97/1200). In Group Two, only 101 samples were obtained. Positive urethral cultures were found in 2 (1.98%), 3 (2.97%) had positive anorectal samples, and 1 (0.99%) was positive for both. Combined incidence was 5.94% (6/101) (NS). Although incidence rose in both groups, it is unknown by a significant difference was found for Group Two women. Serotypes I and II represented 82% of GBS found in Group One and 83% in Group Two. The remainder were serotype III. Neonatal disease occurred in 32/28,189 cases in Group One(1.13/1000) and in 3/1278 in Group Two (2.36/1000) (NS). Fever, chorioamnionitis and prematurity did not significantly associate with GBS colonization. GBS, however, is becoing a frequently isolated pathogen in this setting.