Two methods of determining the virulence pattern of oat crown rust, Puccinia coronata f. sp. avenae, populations collected in 1965, 1966, and 1967, in Eastern and Western Canada were compared. The results with a single-pustule isolate from each of 50 field collections were as accurate as those obtained with two single-pustule isolates from 100 or more field collections. The continued use of Bond and Ukraine as differential host varieties is of little value except for purposes of race identification, because they are attacked by most of the isolates.