Two soybean [Glycine max (L.) Merr.] plant introductions (PIs) were found possessing antibiotic resistance to the soybean aphid (Aphis glycines Matsumura)in early maturing germplasm in an earlier study. Plants with antibiotic resistance can negatively interfere with the reproduction of the aphid and can control the insect effectively. Information on the number of major genes controlling antibiotic resistance to soybean aphids is essential to design efficient breeding schemes to develop soybean varieties with antibiotic resistance to the insect. The objectives of this study were to determine the number of major genes that underlie the antibiotic resistance to soybean aphids in a resistant PI using an F2 population with 460 individuals. Both phenotypic and genotypic data were used to determine the number of major genes involved in antibiotic resistance. Phenotypic data was obtained from greenhouse by inoculating two aphids on young trifoliates of each plant. The plants are then visually rated for damage severity weekly from two to five weeks after inoculation. This will help identify the resistant individuals from the populations and generate data for genetic analysis of the resistance. Over 100 polymorphic simple sequence repeat (SSR) markers were used to genotype the population.