As the result of a study of the relative efficiency of 18 differential oat varieties grouped into (a) the standard 13 differential varieties, (6) a new set containing Anthony, Victoria, Appler, and Bond of the old set plus Landhafer, Santa Fe, Ukraine, Trispemia, and Bondvic, and (c) the combined set of 18, in identifying collections of crown rust (Puccinia coronatavar. avenoe) present in Iowa in 1950, the author gave preference to the new set plus Saia which was added to both new and combined sets in 1951. The mode of inheritance of reaction to races 1, 45, and 101 was studied in several oat crosses. In every case the resistance of Landhafer was apparently governed by a single gene although the degree of dominance varied considerably. Assuming that Early Santa Fe and Santa Fe-Clinton carried two genes for resistance linked with a recombination value of approximately 23 per cent., resistance to races 45 and 101 in three crosses appeared to be inherited in the same manner. The resistance of Trispemia to races 1 and 101 appeared to be governed by a single gene and, as with Landhafer, the degree of dominance varied in the different crosses. Landhafer X Trispemia and x Santa Fe-Clinton exhibited transgressive segregation for resistance to race 101, certain F2 plants being more resistant than either parent.