Necrotising fasciitis and necrotising myositis are rare but serious life threatening conditions reported mainly in human beings and dogs. Most cases to date have been caused by β-haemolytic streptococci of Lancefield groups A, B, C or G. Necrotising fasciitis has been reported only twice in cats and necrotising myositis has never been described. This paper describes a fatal case of necrotising fasciitis and myositis with pneumonia and septicaemia in a nine-year-old cat. The cat had been undergoing treatment for a suspected tear of the cranial cruciate ligament, but on the seventh day of treatment it suddenly deteriorated and died. On postmortem examination, there was an area of hair loss from its left hindlimb and discoloration of the underlying fascia and biceps femoris muscle. Severe necrotising fasciitis and myositis, with numerous intralesional Gram-positive coccoid bacteria, was diagnosed histologically. Other findings included necrotising pneumonia, pleuritis, focal encephalitis, myocarditis and nephritis. Culture of the affected tissues yielded a pure, heavy growth of Streptococcus canis.