A clinal female flight polymorphism exists in the gypsy moth, Lymantria dispar L., where female flight diminishes from east to west across Eurasia. A Russian population where females are capable of sustained ascending flight and a North American population with females incapable of flight were crossed: parentals, reciprocal F(1) hybrids, double reciprocal F(2) hybrids, and all possible backcrosses to both the parental lines were compared. Heritabilities were estimated using a threshold model, female offspring on female parent regressions, and joint-scaling analyses. Heritability of female flight capability measured using a free flight test was at least 0.60, and variation in wing size, muscle strength, and flight behaviors contributed to the flight polymorphism. Relative wing size varied continuously and had a heritability of 0.70. Environmental variation accounted for >90% of the variation in female preflight weight and relative flight muscle strength, as estimated by an inverted female's ability to right herself. Preflight walking behavior and early deposition of eggs were each inherited through a single gene with two co-dominant alleles. There was no evidence for sex-linkage or maternal effects in female flight capability or associated traits. Continued vigilance to exclude and eradicate introductions of strains capable of female flight in North America is warranted even in areas where no females fly, because some of the alleles needed for full flight capability may not be present in the North American populations, and some flight capability is maintained in the hybrids that could increase the rate of spread of L. dispar.