Mastitis caused by coagulase-negative staphylococci (CNS) has recently increased in Finland and in other countries. CNS -infection usually causes subclinical or mild clinical mastitis. Increased somatic cell count in milk results in loss of income for farmers. Milk production of the infected quarter may be reduced. CNS -mastitis is especially common in heifers after fi rst parturition. The udder can be infected even before heifers are bred. CNS -infection may protect the udder against major pathogens, but more research is needed to confi rm this. The most common bacterial species isolated from CNS-mastitis in cattle are Staphlylococcus chromogenes and S. simulans. Aetiology and pathogenesis of mastitis caused by different CNS species may vary, but studies on this are scarce. According to published results, cure rates for CNS mastitic cows treated with antibiotics have been 60-90 percent, and 15-72 percent for untreated cows. CNS infection can persist in the udder without increasing milk somatic cell count substantially, which can enable bacteria to infect other cows. The most important means of preventing CNS mastitis are decreasing the infection pressure in the herd and improving immunity of the cow.