Bioenergy grass species are a renewable energy source, but their productivity has not been fully realized. Improving photosynthetic efficiency has been proposed as a mechanism to increase the productivity of bioenergy grass species. Fluctuating light, experienced by all field grown crops, is known to reduce photosynthetic efficiency. This experiment aimed to evaluate the photosynthetic performance of both C3 and C4 bioenergy grass species under steady state and fluctuating light conditions by examining leaf gas exchange. The fluctuating light regime used here decreased carbon assimilation across all species when compared to expected steady state values. Overall, C4 species assimilated more carbon than C3 species during the fluctuating light regime, with both photosynthetic types assimilating about 16% less carbon than expected based on steady state measurements. Little diversity was observed in response to fluctuating light among C3 species, and photorespiration partially contributed to the rapid decreases in net photosynthetic rates during high to low light transitions. In C4 species, differences among the four NADP-ME species were apparent. Diversity observed among C4 species in this experiment provides evidence that photosynthetic efficiency in response to fluctuating light may be targeted to increase C4 bioenergy grass productivity.