Downy mildew, caused by the pathogen Hyaloperonospora parasitica, is a severe disease of oilseed rape (Brassica napus) seedlings in some regions of Australia. Sixty-three cultivars of Australian spring-type oilseed rape were evaluated for their levels of resistance to five isolates of the downy mildew pathogen, using a cotyledon infection test under controlled-environment conditions. A high level of resistance, characterised by the absence of disease symptoms or only the appearance of very sparse sporulation on inoculated cotyledons, was expressed in cvv. Pioneer 45Y77 and Pioneer 46Y78. This is the first study to identify Australian genotypes of oilseed rape highly resistant to H. parasitica. The resistance to H. parasitica identified in this study will not only enable Australian oilseed rape breeders to incorporate resistance to H. parasitica into new cultivars for enhanced resistance to this disease, but will also allow direct deployment of the most highly resistant genotypes identified directly in situations and regions most conducive to the development of severe downy mildew disease.