Fitness costs associated with the emergence of virulence (loss of avirulence) have been a subject of much debate in plant pathology. Here, differences in fitness between two pairs of wild types and spontaneous virulence mutants in Puccinia striiformis were studied. The mutants differed from their respective wild types in virulence corresponding to the Yr2 resistance gene in wheat. The wildtype and mutant pairs represented different genetic lineages and virulence pheno-types. Colony size, latent period, lesion growth rate and uredinium density and size were assessed on susceptible and Yr2 resistant wheat varieties. No significant difference was observed for any of the evaluated parameters within wild-type and mutant pairs, i.e. the results did not support a hypothesis of a fitness cost for acquiring virulence in P. striifor-mis. In contrast, significant differences were found for most of the parameters when isolates were compared according to genetic lineage, which confirm that the applied methodologies were appropriate for detecting differences in pathogen fitness.