Isolates of the pathogenic ascomycete Leptosphaeria maculans have been cultured from blackleg-affected oilseed rape (Brassica napus) stubble from Horsham, Victoria. These isolates are indistinguishable on the basis of morphological characters, but can be classified as either aggressive or non-aggressive by their ability to infect B. napus cultivars Midas and Westar. These aggressive and non-aggressive isolates of L. maculans can be distinguished by molecular techniques including electrophoretic karyotyping, Southern analysis of the ribosomal RNA gene repeat, Random Amplified Polymorphic DNA (RAPD) marker analysis, and pigment production. The presence of aggressive and non-aggressive strains of L. maculans in North America and Europe has been previously described. This is the first report of non-aggressive L. maculans strains isolated from B. napus in Australia.