Reproductive compatibility of interstrain matings in plum curculio, Conotrachelus nenuphar (Herbst), was studied during the summers of 2004 and 2006. There was a unidirectional incompatibility in the cross-matings along a rough north to south transect of the range of plum curculio (4 × 4 two-factorial design). There was significantly lower fertility in West Virginia males mated with New York and Virginia females (40 and 29%, respectively), and Florida males had a significantly lower fertility with Virginia and West Virginia females (46 and 37%, respectively), but Florida females were compatible with males of all four populations. Three northern populations were compatible with each other in all cross-mating combinations (New York, Massachusetts, and New Jersey; 3 × 3 two-factorial design). There was a unidirectional reproductive incompatibility in the combination of New Jersey males with Florida females (47%) and a bi-directional incompatibility between Florida and West Virginia reciprocal cross-mating (26 and 21%, respectively). The pattern of the reproductive incompatibility among geographic populations could help us to distinguish plum curculio strains and to clarify their strain distribution range to enrich our knowledge on their population ecology and biosystematics.