Dissertation (MMED Obstetrics and Gynaecology)Background: Group B streptococcus(GBS) infection is a leading cause of maternal and neonatal morbidity. It is usually associated with many pregnancy related complications such as, urinary tract infections and sepsis for maternal and neonatal. Awareness of maternal and neonatal GBS colonization, predictors and antimicrobial susceptibility is essential in determining the rate of transmission and an antibiotic with minimal selective pressure for resistance against GBS. Despite the prevalence of GBS still, there is few data from developing countries especially in Tanzania on the prevalence, predictors and antimicrobial susceptibility of GBS among pregnant women and their newborns. Objectives: The main objective of this study was to determine the prevalence, predictors and antimicrobial susceptibility pattern of GBS colonization among pregnant women and their newborns at Iringa Regional Referral Hospital. Methods: This study employed a cross sectional study design, it was conducted between March 2020 and July 2020. Vaginal swabs were taken from 131 women in labor, thereafter the swabs were also taken from their respective neonates’ ear canals and umbilicus immediately after delivery. The swabs were then transported to laboratory for culture and antimicrobial susceptibility. Data analysis was done using a computer software known as Statistical Package for Social Sciences( SPSS) version 20. Results: The prevalence of GBS colonization among pregnant women giving birth at gestation age of ≥37 weeks and their newborns was 23% and 8 % respectively. Hypertension during pregnancy was statistically significantly associated with maternal GBS colonization as mothers who had hypertension were eleven times at risk of GBS colonization compared to those who were not hypertensive (AOR=11.433, 95% CI=2.721-48.038, p=0.001). Multigravidas were fifteen times at risk of GBS colonization compared to prim gravidas (AOR=15.126, 95% CI=3.921-58.353, p=0.000) Antibiotic resistance was as follows: ceftriaxone was 60%, gentamycin 57% and erythromycin 47% resistant respectively. Conclusion: This study found that there is high prevalence of maternal GBS colonization as well as high rates of vertical transmission of GBS to newborns. Moreover, there is high resistance to commonly used antibiotics due to irrational use of the antibiotics.