The inheritance of resistance to blackleg (caused by Leptosphaeria maculans) was examined in the F-1 and F-2 of a cross between highly resistant canola 'Surpass 400' and susceptible 'Westar' in the field. Blackleg-infected canola straw was collected from the field and scattered among plants to increase disease development with the aid of natural rainfall. Disease severity on seedlings was assessed as the average number of lesions on leaves 1 and 2, and on adult plants as the percentage necrosis on a cross-section of stems immediately above the crown. All 'Westar' plants were susceptible (S) and all 'Surpass 400' and F-1 plants were resistant (R) at both growth stages. Disease severity on F-2 plants segregated 3 : 1 (R : S) as expected for a single dominant resistance allele in both the seedling and adult plant stages. There was a high proportion (91.1%) of matching reactions (R-R and S-S) between seedling and adult plants. 'Surpass 400' is the source of a single dominant allele for blackleg resistance in Brassica napus that is expressed strongly in both seedlings and adult plants.