For the first time comparative study was conducted of the ultrastructure of promastigotes of Leishmania major and L. tropica in the organism of Phlebotomus papatasi which for the first species of Leishmania is and for the second species is not a natural invertebrate host. During the bloodsucking on Mesocricetus auratus experimentally infected with Leishmania sandflies perceive amastigotes of both L. major and L. tropica. In the midgut of the sandfly both species of Leishmania pass into the stage of promastigote (therefore, this phenomenon is not defined by species-specific factors). However, further fate of promastigotes of L. major and L. tropica in Ph. papatasi is different. L. major has its normal cycle of development in the intestine of the sandfly and preserves its all ultrastructural peculiarities. Promastigotes of L. tropica, on the contrary, display in the midgut of Ph. papatasi signs of degeneration which are expressed in break of their normal ultrastructure (changes in the shape of cells, break in cell division, thickening or thinning of intracytoplasmatic formations, especially of mitochondria, intensified vacuolisation of cytoplasm, changes in the structure of nucleus and kinetoplast; and others). All these data indicate in an indirect way that the intestine of Ph. papatasi is a favourable medium for L. major and unfavourable one for L. tropica.