Current systems that describe the virulence phenotype in Puccinia graminis f. sp. avenue lack a systematic approach for the naming of races or to provide easily made comparisons of virulence among races. Anew nomenclature system that simply and systematically characterizes virulence in P. graminis f. sp. avenae is described. The new system has the distinct advantage of providing easily seen relationships among races in contrast to previous nomenclature systems. This allows for easier interpretation of virulence relationships in the oat stem rust population and provides a large amount of virulence information with a minimum of written characters. This system uses single-gene differential lines with the resistance genes Pg1, Pg2, Pg3, Pg4, Pg6, Pg8, Pg9, Pg10, Pg12, Pg13, Pg15, and Pg16, grouped into three subsets of four lines in sequential Pg gene order. By grouping in sequential gene number order, the relationship of the new system to the "standard" system is easily seen. Each race is designated by a three-letter code, based on the seedling reaction (low or high) on 12 differential lines. The letter code nomenclature system is open ended and can be updated easily as new differential genes are identified. This system simply and precisely describes the virulence phenotypes of isolates of P. graminis f. sp. avenue, and allows for easily made comparisons of virulence of isolates collected over time and across geographical locations worldwide.