In addition to their growth in the midgut and foregut of the sandfly vectors, parasites of the Leishmania braziliensis complex undergo development in the hindgut, where they are predominantly attached to the wall of the pylorus and, to a lesser extent, the ileum. Such development is absent in the life-cycle of members of the Leishmania mexicana complex, which develop only in the midgut and foregut. This difference was maintained in the sandfly Lutzomyia longipalpis experimentally infected with a variety of leishmaniae, some of which have also been studied in their natural vectors. Development in this laboratory-bred insect therefore serves as a quick means of distinguishing between isolates of the mexicana and braziliensis complexes. Subspecies of Leishmania hertigi, from neotropical porcupines, were not found to develop in the hindgut of Lu. longipalpis: for this and other reasons they are removed from the L. braziliensis complex and placed in a group of their own. Leishmania enriettii, of the guinea-pig, similarly failed to develop in the hindgut: this, with additional features of the parasite, supports its continued inclusion in the L. mexicana complex. The significance of hindgut development of certain leishmaniae is discussed, particularly with regard to the evolution of the genus Leishmania.