A 42-year-old male was admitted to our hospital because of high grade fever on October 6, 1992. He had no history of cardiac and underlying disease. For the past 10 days, he had complained of high grade fever and noticed arthralgia on his left shoulder. Physical examination on admission revealed that there was a body temperature of 39.0 degrees C and tenderness in the left shoulder. There were no abnormal findings for the chest or abdomen. On the second hospital day, he developed a diastolic murmur which had not been present on admission. And blood culture was positive for Streptococcus agalactiae. Ultrasonic-cardiogram indicated the presence of vegetation. He was diagnosed as infective endocarditis and treated with PCG 20 million units/day, IPM/CS 2 g/day and ISP 400 mg/day. But he was not responding to the chemotherapy. Aortic valve replacement was done on 22nd, October. Valve surgery succeeded and he became well after that time. Endocarditis caused by S. agalactiae is extremely rare, and is an important condition which carries a high mortality. Only seven cases of S. agalactiae endocarditis have been reported in Japan. It is difficult to treat these cases with antibiotic therapy alone. Therefore, we suggest that early surgery should be considered in infective endocarditis caused by S. agalactiae.