A comparative study of hydrogen peroxide (H202) generation, the character of a fungal catalase gene expression, and the catalase activity in wheat plants, infected with Septoria nodorum Berk. strains differing in their aggressiveness, has been carried out. The decreased intensity of H202 accumulation in infected tissues, influenced by an aggressive S. nodorum strain and caused by the enhanced transcriptional activity of the fungal catalase gene and the heightened synthesis of its product, has been revealed to be more expressed compared to a similar decrease influenced by a less aggressive strain. An assumption was made that the expression activity of the catalase gene and, therefore, the activity ofcatalase involved in the regulation of the H202 content in the infected zone represent important factors providing high.aggressiveness and pathogenicity of S. nodorum.