Virulence of isolates of Puccinia coronata collected during 1992 to 1998 from Sonora, Chihuahua, Nuevo Leon, and five states in Central Mexico were compared on a set of 27 differential oat (Avena sativa) lines with different genes for race-specific resistance. Frequencies of virulence and the presence of specific pathogenic races were compared among the four regions of Mexico and between Mexico and the adjoining states of California and Texas in the United States. The P. coronata populations in Mexico were highly diverse even though the sexual stage of the fungus is not known to occur there. Overall virulence frequencies were most similar between Chihuahua and Nuevo Leon, but there were more races in common between Central Mexico and Chihuahua than between any other pair of regions of Mexico. No races found in Sonora were found in other regions of Mexico. More races found in Texas also occurred in Nuevo Leon than in any other region of Mexico. Mean virulence complexity was lowest in isolates from central Mexico; greatest in Sonora, California, and Texas; and intermediate in Chihuahua and Nuevo Leon. Significant (P < 0.05) associations of virulences occurred for 24 pairs of virulence genes in at least three of the four regions of Mexico. Virulences to 19 of the 24 pairs were also significantly associated in Texas; virulences to 13 were also significantly associated in California.