To determine the prevalence of maternal vaginal-rectal colonization of group B streptococcus (GBS) in pregnant women who are between gestational weeks 35th and 37th. Design: prospective prevalence descriptive study. Methods: obstetric patients in a private clinic in Bogota who attended between Spril st and September 30th 2008 were included. Screening of group B strep was performed through vaginal and/or rectal cultures according to groups. In the first group vaginal swabs were taken and in the second one vaginal rectal swabs were done. Data from patients whose cultures were positive were identified and anatomical site was identified. Finally, prevalences were determined as well as incidence of early newborn infection within the sample of the study. Results: a sample of 1000 pregnant women was included. 72 women were excluded for not complying with inclusion criteria. Mean age was 27.6 years (SD: ± 1.1; range 35 - 41), mean parity 1.8 (SD: ± 1.0; range 1 - 10). There were no differences between maternal age, parity and colonization with GBS. From screened women only with vaginal cultures, 1.7% presented with positive cultures for GBS. Women screened with vaginal rectal a total of 16.4% presented as positive. Every woman identified as positive was medicated with prophylaxis and no early neonatal infections were detected. Conclusions: prevalence of vaginal-rectal colonization with GBS between gestational weeks 35th and 37th was of 16.4%. Screening through vaginal swabs identified a mean of 1.7%.There were no identified cases of early neonatal infection.