The Hessian fly, Mayetiola destructor (Say), has historically been a major pest of wheat, Triticum aestivum L., in Kansas. Resistant cultivars are used to protect seedling wheat plants from damage by Hessian fly larvae in the fall. However, it is not known if the cultivars are resistant after winter vernalization. Experiments in a greenhouse indicated that ‘Armour', a cultivar considered intermediately resistant, remained resistant under low levels of infestation by Hessian fly, but kernel weight was significantly reduced by a mean of 0.13 g per spike when Hessian flies were abundant. In the field, Armour was not resistant post-vernalization, with infested plants having culm heights reduced by an average of 14.2 cm and kernel weight reduced by 0.10 g per spike compared to noninfested plants. In addition, infested plants had significantly fewer spikelets per spike, and fewer kernels per spike. ‘Duster', a cultivar considered very resistant, was resistant to Hessian fly larvae in the greenhouse and the field, and even produced significantly heavier kernels when Hessian flies were abundant in the greenhouse. Infestation of susceptible cultivar ‘Fuller' in the greenhouse and field resulted in culm heights reduced by up to a mean of 11.2 cm per spike as well as significantly fewer kernels per spikelet. Kernel weight was significantly reduced by a mean of over 0.1 g per spike in both the greenhouse and the field. The results suggested that all cultivars resistant to Hessian fly should be evaluated after vernalization to determine if each continued to be resistant into the spring. Little information is available on if and how feeding by Hessian fly larvae on more mature hard red winter wheat (Feekes 7 to 10) after vernalization impacts plants, aside from lodging. Growers are aware when Hessian fly larvae kill seedlings or cause plants lodging, but small and significant, although less dramatic, yield loss might occur undetected.