Differences in the prevalence and abundance of helminth parasites amongst host populations may result from variations in resistance to infection, differences in habitat preferences, diet or social behaviour. The use of helminth parasites as ‘ecological markers’ for determining differences between morphological groups of wild-living cats in Scotland was investigated, in light of the debate over the definition of a wildcat. The prevalence and abundance of the tapeworm Taenia taeniaeformis did not differ significantly between cats designated as ‘wild’ and ‘feral’ types. The prevalence of both helminths was high (94% and 69%, respectively) and there was significant variation in worm abundance related to season and geographical area. Also, prevalence of infection with Toxocara cati was significantly higher in female cats. Cats which harboured large numbers of one worm species were also likely to harbour large numbers of the other. However, a possible relationship with cat condition was only found for T. taeniaeformis for which there was a significant negative correlation between an index of cat condition and worm abundance amongst ‘wild type’ cats, but not amongst ‘feral type’ cats. Variations in worm burdens could not be attributed to differences in the diet of the cats as there was no significant relationship between the presence of any individual prey type and either the presence or abundance of T. cati or T. taeniaeformis. There was a significant negative correlation between density and biomass of individual worms in infections of T. taeniaeformis, suggestive of a density-dependent constraint on tapeworm growth.