Chitinases (E.C.126.96.36.199) are thought to play an important role in the defense of plants against fungal invasion. By screening a barley genomic library with a previously identified chitinase eDNA clone (clone 10), a genomic clone was isolated and characterized by DNA sequencing of the chitinase coding region and flanking sequences. This clone contains an open reading frame capable of coding for a 34 kD chitinase. Comparison of the amino acid sequence of the encoded protein with other barley chitinases suggests that the genomic clone encodes chitinase T, which has been characterized extensively by protein sequencing. Treatment of barley leaves and aleurone protoplasts with N-acetyl glucosamine oligomers which act as elicitors in other plants, did not lead to the elevation of the levels of the chitinases. However, infection of barley seedlings with the powdery mildew fungus, Erysiphe graminis, resulted in the induction of several isoforms of chitinase. The level and number of chitinase isozymes was correlated with the severity of infection. The infection-related chitinases found in barley leaves are different from those found in seeds.