A nomenclature system for designating virulence phenotypes of Puccinia coronata f. sp. avenae, the causal agent of oat crown rust, is proposed. Sixteen single gene oat (Avena sativa) lines, with seedling resistance genes Pc38, 39, 40, 45, 46, 48, 50, 51, 52, 54, 56, 58, 59, 62, 64, and 68, are the primary differentials. The host lines are arranged into groups of four (subset 1 = Pc40, 45, 46, 50; subset 2 = Pc38, 39, 48, 68; subset 3 = Pc51, 52, 58, 59; subset 4 = Pc54, 56, 62, 64). Avirulence and virulence of Puccinia coronata f. sp. avenae isolates on each line are indicated by low and high infection types, respectively. A letter from the 16 consonants B through T is assigned to each of the 16 possible combinations of low and high infection types on the four differentials of each subset. Designations for P coronata f. sp. avenae virulence phenotypes are indicated by a four-letter code for the virulence combinations on all four subsets, one letter for each subset. Local differential series are separated from the four-letter code by a dash, followed by an additional letter code describing the virulence combinations of the isolates on the local supplemental differentials. When fewer than four differentials are used in the supplemental series, virulence combination on these differentials for each isolate is described by a listing of the Pc gene(s) to which the isolate was virulent, following the four-letter code and a dash. This nomenclature system for P coronata f. sp. avenae is similar to the nomenclature system in use for P graminis f. sp. tritici and P triticina.