There is limited information on the mechanisms of resistance in barley, Hordeum vulgare L., to the Russian wheat aphid, Diuraphis noxia (Mordvilko). 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 (98BX 28-58B) and susceptible (Otis) barley through chlorophyll fluorescence kinetic measurements and carbon assimilation (A/C_i) curves taken at 24 h, 3, 6, 10, and 13 d after aphid introduction. No significant differences in fluorescence parameters were observed between infested and control plants for both barleys. A/C_i curves showed that D. noxia feeding negatively impacts the photosynthetic capacity in both barleys, but this effect was greater in the susceptible barley. A/C_i curves also showed that some level of compensation occurs in resistant barley at day 10. These results demonstrate that aphid feeding has minimal influence on the photochemical efficiency/light reactions of the plant. 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, in both resistant and susceptible barley.