The distribution of native C and C grasses in a temperate arid region of Mendoza, Argentina, was studied in six areas at different altitudes. C species predominate at low elevations in both relative species abundance and plant cover. At high elevations C species are dominant in cover and composition. At medium altitudes (1100-1600 m) grass species composition is balanced but plant cover of C species is greater. Of 31 genera in the whole area, 19 were C. Only the genera Stipa (C) and Aristida (C) were present in all the six areas surveyed. The pattern of grass distribution shows high correlation with evapotranspiration and temperature parameters, but low correlation with rainfall. The relation between grass distribution and different climatic parameters is discussed.