ABSTRACT Genetic variation in field populations of Heterodera glycines is a key issue for both resistance gene deployment and basic understanding of virulence-gene flow in populations. In this study, we examined phenotypically defined genes for virulence under selection from host resistance. We separated the most common H. glycines genotypes in the United States into two virulence groups, based on their reproductive abilities on the resistant soybean plant introduction (PI) 88788. These groups correspond to previously identified virulence genes in the nematode, as follows: the dominant gene in H. glycines to PI88788, and the recessive genes to PI90763 and Pickett/Peking. Virulence allele frequencies and virulence genotype frequencies of selected field isolates were investigated by testing the host range of single-female-derived lines, which were developed through single-female inoculation on the standard susceptible soybean 'Lee 68'. By comparing virulence genotype frequencies between the original field isolates and their single-female-derived lines, we were able to determine allele frequencies in the field populations. The results suggest that tremendous variation in H. glycines virulence genes exists among field populations. Potential mechanisms of selection which could cause virulence genotype frequency increases are discussed as related to population genetics equilibrium theory.