Group B Streptococcus (GBS, S. agalactiae) is known to be the leading cause of neonatal sepsis and meningitis and the infection has been increasingly noted in adults, particularly in those with underlying diseases. Penicillin G is the drug of choice for GBS infection. However, the MIC of penicillin for GBS is greater than that for S. pyogenes. Therefore some GBS infections may be difficult to be treated. However, in Korea, our knowledge on GBS infection is limited. We observed 7 cases of GBS bacteremia during 1993-1996 in a hospital. Of the 7 patients, 3 were less than one month of age with no known underlying disease and 4 were adults with liver cirrhosis or malignancy. One adult patient developed disseminated intravascular coagulopathy and expired. Among the GBS isolates, 4 were serotype III and 3 were Ib. All of the isolates were susceptible to ampicillin, teicoplanin and vancomycin, but most were intermediate or resistant to clindamycin, erythromycin or tetracycline. It is concluded that GBS also cause severe infections in adults with underlying diseases and the serotypes III and Ib may be more virulent than other serotypes. Early detection and antimicrobial susceptibility test of GBS from severe infection may be necessary for the proper treatment of the patients. (Korean J Clin Microbiol 1999;2:220-224)