Few effective genes are available that confer resistance to prevelent North American races of oat stem rust. Lines derived from Amagalon, a synthetic hexaploid line developed from a cross between Avena longiglumis Dur. (2n=14)and A. magna Murphy et Terrell express resistance to stem rust race NA67. We evaluated the F₂ segregation of resistance to NA67 in nine populations derived from crosses of ND990232, a line derived with resistance from Amagalon, and one other Amagalon-derived line, crossed with various NA67 susceptible genotypes. Nine segregating F₂ populations of 100 plants each, produced from crosses involving Amagalon-derived NA67 resistant lines with diverse NA67 susceptible lines, were evaluated in the seedling stage after inoculation with NA67. Eight of the populations fit a 3 resistant (R):13 susceptible (S) ratio, but did not fit 1R:3S or 1R:15S ratios. Homogeneity of error allowed combining the populations and the combined population fit a 3R:13S ratio but did not fit other tested ratios. The data suggest the Amagalon resistance is conferred by one dominant and one recessive gene through dominant suppression epistasis. F_2:3 lines derived from the seedlings evaluated in the greenhouse were evaluated in the field and verified the results obtained in the seedling tests. While resistance of pg-a is conferred by the presence of at least two homozygous recessive genes, resistance of Amagalon-derived lines appears to involve a dominant gene.