From 1952 to 1956 the races of crown rust, Puccinia coronata Corda f. sp. avenae Erikss., that predominated in Eastern Canada were 201, 202, 203, 209, 239, and 240. Races 201, 202, and 204 occurred most frequently in Western Canada. These races cannot attack the differential host variety Victoria, from which the currently grown commercial varieties Rodney and Garry derive their resistance. Garry and Rodney became widely grown in Canada by 1957, and since then races 216 and 274, which can attack Victoria, have become prominent, especially in Western Canada.More races are found in Eastern Canada than in the west, and there is less tendency for a few races to predominate in the east. These differences may be explained by the relatively frequent occurrence of the alternate host, Rhamnus cathartica L., in important oat-growing regions of Eastern Canada, and by the cultivation of large acreages of a few varieties with similar resistance in the west. The most important factor affecting the distribution and frequency of occurrence of races is the variety grown.