Barley, a collateral host of Pyrenophora tritici-repentis (cause of tan spot of wheat), could promote genetic variation in the fungal population and serve as a reservoir of local primary inoculum for a wheat crop in the next season. Samples of diseased barley leaves were collected from 35 locations in North Dakota during the 1999 growing season. P. tritici-repentis was recovered from 2 to 5% of the sampled lesions taken from three of the samples. The majority of the samples also harbored Pyrenophora teres, Stagnospora nodorum, and Cochliobolus sativus. All 10 isolates of P. tritici-repentis recovered from the samples were grouped as race 1, because they induced both necrosis and chlorosis on the appropriate wheat differentials. In addition, 12 barley cultivars were tested for their reaction to five races of P. tritici-repentis, and all were resistant. The fungus could not be recovered when isolations were later attempted from the inoculated leaves. Barley cultivars also were infiltrated with two host-specific toxins, Ptr ToxA and ToxB. They were insensitive, because they did not develop necrosis or chlorosis to the toxins. Our results indicate that barley is highly resistant to the fungus and is unaffected by its host-specific toxins. Prevalence of race 1 in a very low frequency on barley during the growing season suggests that barley does not play a significant role in tan spot epidemiology on wheat or promote variation in the fungal population.