High light intensities such as those found during the middle of the day in southern Africa are not responsible for inhibition of acetylene reduction by mats of blue-green algae in kikuyu lawns. Similar results were obtained with uni-algal cultures of Nostoc isolated from the algal mats. Short term experiments (3 h) with both cultures and algal mats showed a direct relationship between acetylene reduction and light intensities of up to 323 lux but no significant increase in acetylene reduction for light intensities above this. The amount of acetylene reduction by algae in the dark is determined by the light intensity received prior to darkness.