Group B streptococci (GBS) have been recognised for more than three decades as a serious cause of perinatal morbidity and neonatal mortality. The aim of this study was to accurately determine the prevalence of GBS carriage and the serotype distribution among pregnant Irish women. 504 women attending antenatal clinics had two swabs (one perianal and one low vaginal) taken in the last four weeks of their pregnancy. These were placed in Todd Hewitt broth and then subcultured onto solid media. Serotyping of the isolates was performed by the Central Public Health Laboratory, London. GBS colonised women were treated with prophylactic antibiotics in labour and their infants received prophylaxis for 48 hours. 129 women (25.6%) were found to be asymptomatically colonised with GBS. Dual site screening (low-vaginal and perianal) identified 5% more GBS carriers than one site would have done. Serotypes identified included types I (30%), II (17%), III (30%), IV (1%) and V (9%). GBS colonisation is very common in Irish pregnant women and therefore a strategy for management in pregnancy ought to be developed in order to reduce the recognised occurrence of neonatal morbidity and mortality caused by this organism.