In grasslands composed of C3 and C4 plants, a different pattern of response by the two photosynthetic types to fertilizer may change the floristic composition of the community. In this study, we evaluated the effects of an application of nitrogen (N) and phosphorus (P) fertilizer at two dates in the spring on the aerial growth of C3 and C4 grasses. Danthonia montevidensis and Stipa neesiana(C3), and Setaria geniculata and Sporobolus indicus (C4), were selected from a native grassland of the Flooding Pampa, Argentina. Two consecutive experiments (early and late spring) were conducted in pots filled with the local soil. The C3 grasses showed no response in aerial biomass, and only minor responses in aerial concentrations of N and P, numbers of leaves and tillers, and nutrient use efficiency to an application of P or N fertilizer. In contrast, their C4 counterparts showed a consistent positive response. C4 species performance was considerably influenced by date of fertilizer application in the spring. There was on average a 1·8 and a 2·5 increase in aerial biomass in early and late spring, respectively, to fertilizer application. The later date in spring was associated with higher ambient temperatures and it appeared that temperature rather than fertilizer application was the main limiting factor for C3 species. It was concluded that the positive response in C4 species to an application of P and N fertilizer indicates that the differences between C3 and C4 species were related to a higher demand to sustain greater growth rates in C4 species.