Infection processes were examined to investigate the breach by a strain of Leptosphaeria maculans of anatomical barriers in cv. Surpass 400, a cultivar containing single dominant gene-based resistance (SDGBR). Two strains, UWA 192 and UWA P11, were used to inoculate cvs. Surpass 400 and Westar. The pre-penetration and penetration behaviour of both strains was similar in both cultivars. However, they differed significantly after penetration. When UWA P11 infected cv. Surpass 400 through stomata, guard cells rapidly died within a few hours and the surrounding mesophyll cells became necrotic, constituting a hypersensitive reaction (HR). Hyphal growth continued, albeit slowly, through the intercellular palisade mesophyll and spongy mesophyll spaces, but hyphae rarely spread beyond the HR region, and did not sporulate. Polyphenolic compounds accumulated in the area bordering the HR. However, when UWA 192 infected through stomata, symptoms were not evident until 10-12 days postinoculation (dpi) and were typically characterized by pale tan to grey circular lesions in which abundant pycnidia were produced by 14 dpi. Subsequently, hyphae extensively spread beyond the lesion border, reaching the veins and progressing down the petiole towards the stem. Where the SDGBR remained effective (i.e. against strain UWA P11) death of cells was restricted to a few palisade cells within the HR, even though hyphae were present in the lower tissue layers of the cotyledon. In contrast, where the SDGBR was not present (cv. Westar) or was overcome (cv. Surpass 400 with UWA 192), extensive death of epidermal and upper and lower palisade cells occurred throughout infected areas of the cotyledon, with subsequent abundant production of pycnidia. Polyphenolic compounds, which are also associated with resistance, did not accumulate in this instance. It was evident that the ability of the host to instigate the HR mechanism displayed by cv. Surpass 400 was lost with UWA 192 resulting in a ''normal'' susceptible response. This is the first study of the specific processes involved in the breaching of the HR in cv. Surpass 400.