||Plant species that fix CO2 by the C4 cycle have higher rates of CO2 uptake than species using the C3 photosynthetic carbon reduction cycle. Greater CO2 fixation capacity has been associated with reduced photorespiration, specialized leaf anatomy, and biochemical pathways that differ in C3 and C4 plants. The higher photosynthesis rate of C4 species also results in more dry matter production per unit of water transpired. This paper reviews published reports of productivity and N content of some C4 and C3 species. It hypothesizes that C4 plants have a greater N use efficiency (biomass production per unit of N in the plant) than do C3 plants. This difference presumably results from the relatively smaller investment of N in the photosynthetic carboxylation enzymes of C4 plants than of C3 plants. Some adaptive and evolutionary implications of such a hypothesis as well as limitations of supporting data are discussed.