New iminosugars (1-oxabicyclic β-lactam disaccharides) have been synthesized as inhibitors of elongating α-d-mannosyl phosphate transferase (eMPT), a key enzyme involved in the iterative biosynthesis of cell-surface phosphoglycans of the Leishmania parasite. The design is based on a transition-state model for this remarkable enzyme that transfers intact α-d-mannosyl-phosphate from GDP-Man. Since these phosphoglycans are unique to Leishmania and are essential for its infectivity and survival, their biosynthetic pathway has emerged as a novel target for anti-leishmanial drug and vaccine design.