Acute necrotizing mastitis in a cow was investigated pathologically. The udder was firmly enlarged to twice the size of a basketball. Histologically, the epithelium of the interlobular duct of the mammary glands was displaced by necrotic debris and bacterial clumps (Grampositive cocci). The intralobular ducts were also necrotic. The alveoli showed scattered necrotic foci varying in size and containing bacterial clumps. The vascular walls of the interlobular stroma exhibited fibrinous necrosis with fibrinous thrombi. The liver showed centrolobular fatty and droplet degeneration of the hepatocytes. Extramedullary hematopoiesis was observed in the liver, spleen, adrenal glands, lungs and kidneys. The bacteria observed in the lesions of the mammary glands were stained positively with immunoperoxidase using an antibody against Staphylococcus aureus. Ultrastructurally, the bacteria were round or oval in shape and had thick cellular walls. Capsule-like fibrous material was present around the surface of the bacteria. Bacteriologically, Staph. aureus was isolated in pure culture from the mammary gland. These findings suggest that Staph. aureus caused the acute necrotizing mastitis. The pathogenesis of the disease was examined.