The development of Leishamnia infantum, the causative agent of visceral leishmaniasis, was studied by transmission electron microscopy in the midgut of the sand fly Phlebotomus langeroni, a natural vector. The suprapylarian Leishmania behavior was elucidated. The population of L. infantum promastigotes in the midgut was composed of several morphological forms that differ in cell shape, flagellum length, and attachment. Ingested promastigotes multiplied within the bloodmeal to small, oval, and slender promastigotes. The disintegration of the peritrophic matrix occurred simultaneously with the morphological transformation of parasites from procyclic forms to long nectomonads. These transformed into short nectomonads (leptomonads). The long nectomonads and short nectomonads attach to the abdominal and thoracic regions of the midgut in a highly specialized manner. The migration of parasites from the abdominal cardia occurred at the tenth-day-postinfection by different concurrently presented forms. These forms probably represent intermediate forms in the transformation sequence into infective metacyclics. Some morphological and physiological changes were observed in infected females compared to uninfected females. The pattern of variation was advantageous to the parasite, allowing development, establishment, and a continuous reservoir of dividing populations in the midgut.