Net assimilation rates and other growth attributes were compared for rape, sunflower, and maize plants growing widely spaced at temperatures of 10°, 16°, 22°, 28°, and 34° C, in light of 3, 000 f.c. intensity. The optimum temperature for net assimilation rate lay between 20° and 30° C, and was lowest for rape and highest for maize. The temperature coefficient of the net assimilation rate was lower than that of the relative growth-rate, especially in rape and sunflower, corresponding to an increase in leaf-area ratio with in temperature. This arose to an increase in leaf-area ratio with rise in temperature. This mcrease arose through change inleafarea/leaf weight; temperature had little effect on leaf weight/plant weight. In moderate to warm conditions the net assimilation rate varied little with temperature: by only± 10 per cent between 12° and 30° C for rape, and 23° and 36° C for maize. This agrees with observations in natural climates which suggest that temperature is generally less important than light in controlling net assimilation rates, except in cool climates. In natural climates, as in these controlled climates, relative growth-rate is more temperature-dependent.