A Pyrenophora graminea strain expressing the β-glucuronidase gene (GUS) was obtained via genetic transformation, and used to follow the penetration of the pathogen inside barley germinating seeds and the colonization of host tissues. Significant differences between resistant and susceptible barley cultivars were observed in the colonization of artificially-infected embryos by the fungus. These results suggest that the GUS transgenic strain of P. graminea will be useful for the early screening of barley cultivars for resistance to leaf stripe disease.