Technical Abstract: There is limited information on the mechanisms of resistance in barley, Hordeum vulgare L., to the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae). Knowledge of the physiological responses of barley to D. noxia feeding is critical to identifying resistance mechanisms and understanding the defense response of barley to aphid injury. This study documented the impact of D. noxia feeding on a resistant (‘Sidney’) and susceptible (‘Otis’) barley through chlorophyll fluorescence measurements, chlorophyll content, and carbon assimilation (A-Ci) curves taken at 1, 3, 6, 10, and 13 d after aphid introduction. All chlorophyll fluorescence parameters evaluated were similar between aphid-infested and control plants for both barleys. A-Ci curves showed that D. noxia feeding negatively impacts the photosynthetic capacity in both barleys, but this effect was greater in the susceptible plants. From the A-Ci curves, it is apparent that compensation occurs in resistant barley by d 10, but by the conclusion of the experiment, aphid populations reached levels that overwhelmed the resistant barley seedlings. Differences observed in carbon assimilation curves between control and infested plants show that D. noxia feeding negatively impacts the carbon-linked/dark reactions, specifically rubisco activity and RuBP regeneration. It is likely that declines in the photochemical efficiency and chlorophyll content of the plant may be a secondary effect and not the primary trigger of declines in host plant function.