Development and forward migration of Leishmania parasites in the sandfly gut is accompanied by morphological transformation to highly motile, non-dividing 'metacyclic' forms. Previous studies in vitro have demonstrated that this metacyclogenesis is associated with developmentally regulated changes in expression of two major surface glycoconjugates of Leishmania, the lipophosphoglycan (LPG) and the glycoprotein protease GP63. Studies presented here are the first to examine in situ the changes in expression of these two important surface molecules which occur during amastigote-initiated development of L. major in its natural vector Phlebotomus papatasi. Immunocytochemical analysis using a GP63-specific monoclonal (3.8), and others recognizing metacyclic-specific (3F12) and common (WIC79.3) epitopes of LPG on logarithmic and metacyclic promastigotes, demonstrates: (1) clear expression of LPG and GP63 from 2 and 7 days post-bloodfeeding, respectively; (2) developmental modification of the LPG molecule as parasites undergo forward migration and morphological changes associated with metacyclogenesis; and (3) striking deposition of large amounts of parasite-free excreted LPG on/in the epithelial cells of the gut wall.