The impact of the parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck and their herbivorous host the wheat stem sawfly, Cephus cinctus Norton, a pest of wheat Triticum aestivum were investigated for yield in wheat grown in the field. Wheat stem sawfly infested stems had a higher yield potential than uninfested stems before infestation. However, final reproductive output, expressed as mean seed weight per fertile spikelet, was not significantly different between ears on infested stems that supported complete larval development when compared to ears on uninfested stems. Stems containing parasitized larvae and stems containing larvae that died before completing their development had a higher mean number of seeds and seed weight per fertile spikelet than either infested with live larvae and uninfested stems. These results suggest that larval feeding prevented infested stems from attaining their yield potential, and that the negative impact of the pest on wheat yield was reduced when late instar sawfly larvae were parasitized. Even though some feeding occurs before parasitism, early damage has a comparatively low impact on yield. This is the first study to show a yield benefit and enhanced plant fitness due to the wheat stem sawfly parasitoids B. cephi and B. lissogaster. This results from the maintenance of increased seed number and seed weight in the higher yielding stems that are preferentially infested by this pest.