'Mycosphaerella graminicola' (Fuckel) Schroeter (anamorph: 'Septoria tritici' Rob.ex Desm.), the causal agent of Septoria leaf blotch of wheat, has increased in prevalence in Western Canada in recent years. Physiologic variation has been reported for this pathogen in other countries. Isolates collected from bread wheat growing in Manitoba and Saskatchewan were tested for virulence on a set of tetraploid and hexaploid wheat differential lines. Seedlings were inoculated with a conidial suspension and assessed three weeks later for the presence of necrosis, chlorosis and pycnidia to identify virulence patterns. Reactions of the differential host lines to different isolates ranged from susceptible to near-immune. Differences in virulence patterns indicated that two races were present in the pathogen population of Western Canada, suggesting that physiological specialization does occur. Cytological observations of resistant and near-immune reactions suggested that a hypersensitive response occurred in both tetraploid and hexaploid wheat lines as early as two to three days post-inoculation. In these lines, mesophyll cell collapse occurred in areas surrounding the stomatal penetration sites. These cells fluoresced when stained with KOH-aniline blue. There was no evidence of direct penetration of the mesophyll cells. Limited intercellular growth of hyphae did occur in both resistant lines. In the susceptible lines, both tetraploid and hexaploid, extensive intercellular growth of hyphae was consistently observed in the mesophyll tissue. Mesophyll cell collapse and pycnidia formation in the substomatal chambers of infected stomates began eight to ten days post-inoculation. No fluorescence was evident in the compatible reaction, at any of the sampling periods.