SUMMARYGrowth of Indonesian C4and C3grasses was compared in field experiments in Indonesia and in growth chamber experiments in the Netherlands. Two C4species,Axonopus compressus(Swartz) Beauv. andSetaria plicata(Lamk.) T. Cooke, were used and one C3species,Oplismenus compositus(L.) Beauv.Both the field and the growth chamber experiments showed no large differences in relative growth rate between the C3and the C4grasses, grown at high light intensities, neither with a high, nor with a low water supply.Net photosynthesis of the C3species was lower than that of the C4species at all growth and experimental conditions. Whenever this resulted in a lower unit leaf rate of the C3species, this disadvantage was overcome by formation of a larger leaf area.At very low light intensities, the C4speciesAxonopushad a lower relative growth rate than the C3speciesOplismenus. This may explain the absence of this C4grass in the deep shade.The absence of the C3species in open areas cannot be explained by a lower relative growth rate of this species at high light intensities. Other factors as a low shoot/root ratio, a high soluble sugar content and a high initial growth rate may give C4grasses a high competitive value over C3grasses in open and dry areas. Some of these factors may depend indirectly upon the C4mechanism of photosynthesis.