Results of precipitin tests made on the contents of the digestive tracts of Triatomíneos are presented. The implications of the finding in these bugs of mammalian and avian blood, as well as that of certain groups of vertebrates found in association with them, are discussed. The study was made on the following species: Belminus peruvianus, Panstrongylus geniculatus, Panstrongylus herreri, Panstrongylus megistus, Psammolestes tertius, Rhodnius neglectus, Rhodnius prolixus, Triatoma arthurneivai, Triatoma dimidiata, Triatoma infestans, Triatoma protracta, Triatoma rubrovaria and Triatoma sórdida. The following conclusions were made: 1 - Species that feed principally on human blood in artificial habitats, are biting birds and cold blooded animais, a marked preference for mammalian blood is shown. 2 - Species that are principally associated with avian hosts in artificial habitants either show a predilection for avian conditions. 3 - Species associated with cold-blooded animais or with a single mammalian group can colonise houses if they have been previously colonised by the appropirate host. 4 - There were no indications that any of the species studied were becoming adapted to a domiciliary habitat. AH the triatomine species mentioned aiready posses the attributes under natural conditions that are essential for their adaptation to new habitats ocuppied by man or domestic animais.