This study was designed to evaluate the effect of Russian wheat aphid Biotype 1 on yield to determine how pure a resistant wheat cultivar needs to be to avoid significant yield loss. The study was conducted during the 2002–2003 growing season at Akron, Fort Collins, and Lamar, Colorado and consisted of five levels of resistance (0, 25, 50, 75, and 100%) in mixtures of the resistant wheat cultivar, Prairie Red, and the susceptible wheat cultivar, TAM 107. Yield loss was found to be linear, and proportional to percentage of resistant plants. At Fort Collins and Lamar, mixtures with any level of resistance supported fewer Russian wheat aphids than the pure susceptible stand, with the least number of aphids found on the pure resistant stand at the 10X infestation level. At Fort Collins, yields were lower when mixtures contained 0 to 25% of the resistant cultivar at the 1X and 10X infestation levels. Yields at the 10X infestation level were greater in the pure resistant stand. As expected, when Russian wheat aphids are abundant, wheat mixtures with 100% resistant plants yielded most.