We present quantitative information on the natural history of a host-parasite interaction that consists on the holoparasitic mistletoe Tristerix aphyllus (Loranthaceae) and its cacti host species. More specifically, we inquire into the historical and biogeographical setting of the relationship, and quantify the autoecology of the floral biology, pollination, seed dispersal, and parasitism of the system. The impact of the mistletoe on the evolution of defense systems is evaluated both at intraspecific and interspecific levels through consideration of the potential for parasite-mediated selection and the geographical structure of the host-parasite interaction. Finally, we suggest prospective lines of research which include aspects related to: (i) the evolution of virulence, (ii) the geographic structure of the interaction, and (iii) the historical evidence for adaptation. These topics will allow us to gain a better understanding of the ecological subtlety and evolution of this singular interaction in semiarid Chile.