The first cases of the Dutch elm disease in Canada were recorded in 1944 in the Province of Quebec, but there is evidence that the causal organism was introduced around Sorel before 1940. In 20 years the disease has extended over more than half the range of the white elm in the Province, or about 25,000 square miles, with an average rate of spread of some 1,200 square miles per year. Observations in 6 one-square-mile areas during a 13-year period indicate that between 5 and 30 per cent, or an average of 16 per cent, of the elm trees were killed by this disease. On this basis the Dutch elm disease has killed at least 600,000 trees during its 20-year history in Quebec. The relative density of elm trees appears to be the main factor which has determined the intensity and pattern of attack.