Hookworms of the genus Ancylostoma are the most pathogenic parasites of young cats, and A. tubaeforme may cause morbidity or mortality in young individuals of the most endangered felid species in the world, the Iberian lynx (Lynx pardinus). Since the transmission of monoxenous parasites is related to host density and remaining lynx populations are currently very small, the presence of reservoir hosts may be necessary for the maintenance of the hookworm life-cycle, the domestic cat being the most likely reservoir of A. tubaeforme. In order to confirm this hypothesis, the mitochondrial cytochrome c oxidase subunit I gene (Cox I) sequences of three A. tubaeforme specimens from a road-killed Iberian lynx from Doñana were compared with 14 specimens retrieved from five sympatric free-roaming cats from the same area, and with six specimens from three free-roaming cats from the Mediterranean island of Mallorca. Gene fragments (300. bp) from 23 A. tubaeforme individuals representing 16 different haplotypes were obtained. A statistical parsimony haplotype network analysis showed that the three specimens infecting an Iberian lynx corresponded to two different haplotypes, one of which was identical to a specimen in a cat found only 10. km from the lynx. Specimens from the Iberian lynx and those from cats in Doñana were only 1.03% genetically divergent, whereas specimens from Mallorca cats and those from Doñana cats and the lynx diverged by 1.33% and 1.36%, respectively. The existence of shared haplotypes of hookworms between lynx and cat reinforces the hypothesis that the abundant sympatric domestic cat population is acting as a reservoir for A. tubaeforme infection in the endangered Iberian lynx. © 2011 Elsevier B.V.