Hybridization and introgression between species is remarkably common, even between distantly related taxa. This suggests that the frequency of hybridization between species has been greatly constrained, not by evolutionary divergence in isolation, but by lack of opportunity. This constraint is being relaxed by human-mediated dispersal. Here, we document a case where such dispersal of a widespread species of freshwater turtle (Emydura macquarii) into the highly restricted range of a critically endangered endemic turtle (Myuchelys georgesi) has provided op - portunity for the two to hybridize. This has raised concerns about the potential impact of hybridization on the endemic species, and its continued persistence in the face of challenges brought about by habitat alteration, increased competition, disease and genetic pollution. This study serves to highlight the risks associated with human-mediated dispersal, which can bring into contact species that would otherwise never or only rarely have met, and thus provide opportunities for hybridization and introgression between even distantly related species, with uncertain consequences for already threatened species.