A comparison of photosynthetic nitrogen (N) use efficiency between C3 and C4 species within the genus Cyperus was made at a range of available nitráte levels. Net photosynthetic rates (P^) of both types increased with N content but CO2 assimilation on a leaf area basis was consistently higher in C4 species in comparison to the C3 species. C4 plants had a higher photosynthetic N use efficiency than C3 plants, and achieved higher Pn at lower N and protein levels. The higher leaf photosynthetic N use efficiency (PNUE) in C4 plants is due to their high raťher than low N and protein contents. Because C^ leaves contain less N than C3 leaves for a given Py^, PNUE is significantly higher in C4 than C3 plants. Nitrogen supply had highly significant effect on the leaf N concentration, but had no significant influence on the CO2 compensation concentration. Carboxylation efficiency was significantly higher in C4 leaves than C3 leaves of Cyperus species, due to the CO2 concentrating mechanism of C4 photosynthesis.