Group C streptococci are flora members of skin, nasopharynx, gastrointestinal and genitourinary systems. They are rare causes of human pharyngitis, arthritis, pneumonia, meningitis and bacteremia. In this report, a 71-years old male patient with Streptococcus equisimilis arthritis/prosthetic joint infection has been presented. The patient was admitted to the emergency service with the complaints of erythema, swelling and tenderness on right knee which had total knee prosthesis. Examination of synovial fluid punction sample yielded abundant amount of leukocytes (> 1000 cells/mm(3)). Empirical ampicillin-sulbactam (1 g q6h, parenterally) therapy was initiated. Bacteria which have been cultivated from synovial fluid specimen were identified as S. equisimilis. The isolate was found to be susceptible to penicilin, erythromycin and teicoplanin, and resistant to chloramphenicol and tetracycline. Although clinical presentation improved during the first ten days, symptoms recurred after the 10(th) day and the therapy was switched to teicoplanin. The recurrence was thought to be the result of antibiotic tolerence. The patient was treated successfully with teicoplanin, and no relapse or reinfection was observed during one year of follow-up. To our knowledge this is the first case of S. equisimilis arthritis reported from Turkey and first case of S. equisimilis associated prosthetic joint infection.