The genetics of the ability of the Great Plains race and Races A and E of the Hessian fly, Mayetiola destructor (Say), to survive on wheat cultivars ‘Seneca’ and ‘Monon,’ which have different genes for resistance (antibiosis), was studied in 3 crossing experiments. Results of reciprocal crosses between Great Plains race (larvae cannot survive on either cultivar) and Race A (larvae survive on Seneca but not on Monon) and between Great Plains race and Race E (larvae survive on Monon but not on Seneca) showed that the ability of Race A to survive on Seneca and of Race E to survive on Monon are controlled by single recessive gene pairs. The F2 and backcross generations demonstrated that males transmit only maternally derived chromosomes and that paternally derived chromosomes are eliminated during spermatogenesis. Since the F1 males of reciprocal crosses bred as if they were homozygous for the genotype of their female parent, the phenotype of F2 and backcross progenies differed according to the direction of the cross between the F1 parents. In a test for allelism, the reciprocal crosses between Race A and Race E produced F1 progenies that were the phenotype of the Great Plains race. Therefore, the ability to survive on Seneca and the ability to survive on Monon are controlled by genes at different loci and not by alleles of the same gene. Then the genie systems of the insect and host are complementary since there is a gene-for-gene relationship, and each resistant gene in wheat has a complementary gene for survival in the insect.