The most widespread and damaging fungal disease of the oat plant is crown rust. Resistance to the crown rust pathogen, Puccinia coronata Cda. f. sp. avenae (Pca), at the seedling stage of Avena sterilis accessions from the Polish national genebank was characterised by five North American and Polish pathotypes of Pca of diverse pathogenicity. Pca pathogenicity was determined on a series of 34 differential lines carrying known seedling resistance genes. Seventy-five percent of studied accessions showed a heterogeneous infection pattern, 17% behaved as homogenous susceptibles, and 7% of tested genotypes could be unambiguously described as resistant. This study proved that A. sterilis accessions preserved in a genebank as complex populations could be a very valuable source of resistance to crown rust. The complexity of analysed populations was ascertained by a detailed variance analysis of transformed resistance/susceptibility data. We demonstrate here that hidden sources of resistance may be discovered in accessions with general susceptibility.