The fine structure of the promastigotes of Leishmania mexicana amazonensis in the midgut, cardia, oesophageal valve and pharynx of the sandfly Lutzomyia longipalpis is described. Differences were found between the nectomonad and the haptomonad promastigotes. There were fewer subpellicular microtubules in the nectomonads than in the fatter haptomonads; the mitochondrion of nectomonads was a single straight longitudinal ramus while that of haptomonads had a tortuous shape; the kinetoplast and nucleus lay closer together in the haptomonads which also had fewer free ribosomes and lipid containing bodies than the nectomonads; the nucleus was spherical in haptomonads but elongate in nectomonads. The reservoir region in the nectomonads was notably voluminous and the surface was modified for pinocytotic uptake of material through coated vesicles, and also for exocytosis. In both haptomonads and nectomonads, a contractile vacuole was present adjacent to the reservoir; flagellar-parasite desmosomes formed a collar anchoring the flagellum as it emerged from the reservoir; four reservoir-associated microtubules ran parallel to the flagellar reservoir; the basal body, axoneme and paraxial rod were similar in structure to those of other trypanosomatids. Apparently intracellular promastigotes were found in the midgut cells. Preliminary studies of pharyngeal forms showed that they were attached by hemidesmosomes to the cuticular pharynx. These forms were not typical promastigotes but had an opisthomastigote-like configuration with the reservoir running past the nucleus. Differences in the ultrastructure of Leishmania in the sandfly and in culture are discussed, and it is concluded that cultural forms should not be considered identical to those in the vector.