Eleven Central American populations of Lutzomyia longipalpis (Lutz & Neiva) were analyzed for genetic variation at 16 enzyme loci. The aim was to study the genetic structure among populations within this region and to identify demes that may represent different sibling species. Genotypic frequencies within populations agreed with Hardy-Weinberg expectations, indicating that there were no sympatric sibling species among these 11 populations. Levels of genetic distance between pairs of populations were very low (< 0.02). Some substructing was evident, because after genotypes of all populations mere pooled, 7 of the 16 enzyme loci deviated from Hardy-Weinberg expectations. Estimates of effective migration rates among populations (Nm) were low (3.7), indicating that gene flow was restricted. These data explained observed genetic substructuring when all genotypes were pooled.