The natural morphological variation in the wildcat, Felis silvestris, and morphological changes possibly caused by introgressive hybridisation with the domestic cat, F. catus, were examined, based on up to 39 variables concerning cranial morphology. The samples of wild-living cats originated from Scotland and southern Africa and consisted of both classical wildcat and other pelage types. Principal component and cluster analyses suggested that introgressive hybridisation occurred in both areas, with the consequence that the characteristics of local wildcat populations had been altered in terms of the frequencies of occurrence of certain characters, especially those concerning cranial capacity. In both regions the clustering patterns of wild-living cats can be interpreted as containing four main groups. One of these consisted mainly of ‘non-wildcats’ and groups furthest from the ‘non-wild’ cluster consisted of the highest proportion of ‘wildcats’ (c. 80%). We propose that where a population is heavily introgressed, the only feasible way to define a wildcat is on the basis of inter-correlated features and conservationists must take a population-based approach to assess the extent of introgression. This approach may provide an operational standard for assessing the impact of hybridisation between wildcats and domestic cats throughout the species' range; it suggests that the Scottish wildcats may be critically endangered.