Intramural Research Program of the NIH, National Institute of Allergy and Infectious Diseases. A.D. holds the Canada Research Chair on the Biology of Intracellular ParasitismThe major surface lipophosphoglycan (LPG) of Leishmania parasites is critical to vector competence in restrictive sand fly vectors in mediating Leishmania attachment to the midgut epithelium, considered essential to parasite survival and development. However, the relevance of LPG for sand flies that harbor multiple species of Leishmania remains elusive. We tested binding of Leishmania infantum wildtype (WT), LPG-defective (Δlpg1 mutants), and add-back (Δlpg1 LPG1) lines to sand fly midguts in vitro and their survival in Lutzomyia longipalpis sand flies in vivo. Le. infantum WT parasites attached to the Lu. longipalpis midgut in vitro, with late-stage parasites binding to midguts in significantly higher numbers than were seen with early-stage promastigotes. Δlpg1 mutants did not bind to Lu. longipalpis midguts, and this was rescued in the Δlpg1 LPG1 lines, indicating that midgut binding is mediated by LPG. When Lu. longipalpis sand flies were infected with the Le. infantum WT or Le. infantum Δlpg1 or Le. infantum Δlpg1 LPG1 line of the BH46 or BA262 strains, the BH46 Δlpg1 mutant, but not the BA262 Δlpg1 mutant, survived and grew to numbers similar to those seen with the WT and Δlpg1 LPG1 lines. Exposure of BH46 and BA262 Δlpg1 mutants to blood-engorged midgut extracts led to mortality of the BA262 Δlpg1 but not the BH46 Δlpg1 parasites. These findings suggest that Le. infantum LPG protects parasites on a strain-specific basis early in infection, likely against toxic components of blood digestion, but that it is not necessary to prevent Le. infantum evacuation along with the feces in the permissive vector Lu. longipalpis.