The analysis of eight speciemens of Triatoma dimidiata (Latreille, 1811) of diverse origins, in order to characterize the structure of the male external genitalia (endosoma process, struts, phallosoma, vesica) led us to point out that the same variability is demonstrate by the species in relation to its external colour characters and the relation between the head and rostrum. Triatoma dimidiata is a vector of Chagas' disease in several American countries, from Mexico (extreme north of its distribution) to the north of South America, including Colombia, Venezuela, Ecuador and Peru. Similarly we have carried out a large scale study on male genitalia of Triatoma infestans (Klug, 1834) which also has a wide geographic distribution, but with stable external characters. The spines of the endosoma process which generally are few in number, might not exist in a small percentage of cases. We analysed fifteen specimens of diverse origins.