We report the third case of endocarditis caused by the newly described micrococcal species Kytococcus schroeteri. A 49-year-old woman was admitted to the hospital with suspected prosthetic valve endocarditis. Five blood cultures and prosthetic valve cultures grew the same type of organism, initially identified as Micrococcus sp. Assignment to the genus Kytococcus was suggested by the arginine dihydrolase activity and resistance to oxacillin. After sequencing of the 16S rRNA genes, the isolate was recognized as K. schroeteri. The patient was treated first with vancomycin combined with gentamicin and later with pristinamycin and rifampin. Three cases of K. schroeteri endocarditis described within a short period of time might indicate a specific pathogenicity of this new species. The isolation of kytococci from normally sterile sites should not be overlooked. CASE REPORTA 49-year-old woman was admitted to the hospital due to persistent fever for 1 week. She had a history of rheumatic heart disease and had undergone mitral valve replacement 10 years earlier. On admission, the patient denied any recent history of dental work, intravenous drug abuse, or diagnostic procedures. Her temperature was 38.5°C, her blood pressure 110/60 mm Hg, and her pulse 88 beats/min. A systolic murmur was heard on cardiac auscultation, and the lungs were clear by auscultation and percussion. The findings of the rest of her physical examination were unremarkable. Laboratory tests revealed the following values: hemoglobin, 120 g/liter; white blood cells, 8.4 ϫ 10 9 /liter; erythrocyte sedimentation rate, 100 mm/h; and C-reactive protein, 125 mg/liter. The liver function tests, renal function tests, and serum electrolyte levels were all normal.A transesophageal echocardiogram disclosed prosthesis disinsertion, with two voluminous and mobile vegetations involving the mitral prosthesis (sizes, 12 by 13 mm and 12 by 12 mm). Mitral valve replacement was recommended. Six sets of blood cultures were drawn. Therapy with cefotaxime and amikacin was instituted, and valve replacement surgery was performed. Five blood cultures and prosthetic valve cultures yielded gram-positive cocci. According to the antimicrobial susceptibilities of the isolates, the therapy was replaced with vancomycin-gentamicin and later with pristinamycin-vancomycin, and our patient responded well to this combination therapy. Six weeks after admission, the patient was discharged from the hospital in good condition and continued to take oral rifampin and pristinamycin for 3 weeks.Microbiology. Blood cultures were processed with a BacT/ Alert system. In five of the six blood cultures drawn on admission, growth was detected after an incubation of 72 to 94 h. Subcultures on blood agar yielded circular, convex, smooth, muddy yellow, and nonhemolytic colonies of heterogeneous size after 24 h of incubation. These colonies increased in size to about 1.5 to 2.5 mm in diameter after 48 h. A Gram stain of these colonies showed spherical cells, predominantly occurring in pairs, in tetrads, a...