Natural populations of nitrogen-fixing cyanobacteria were studied in five Colorado reservoirs. The objective of the study was to measure the light response of nitrogen fixation and the importance of light history to the light response. The study revealed that nitrogen fixation in Aphanizomenon, a heterocystous cyanobacterium, is adapted to low light availability. Nitrogen fixation became light saturated at irradiances below 100 μmol photons m^-2 s^-1. Inhibition at high irradiances was minimal or absent; nitrogen fixation did not become inhibited when average irradiance reached 1000 μmol photons m^-2 s^-1. Aphanizomenon also is capable of considerable fixation in the dark (29% of gross rate) and is more responsive to increases in light intensity than Anabaena and less dependent on past light history. Previous light exposure had an effect on both gross and dark nitrogen fixation rates in both genera. Only light fixation rates in Anabaena were affected by previous light exposure. Because of its ability to saturate rapidly in the light and continue at considerable rates in the dark, Aphanizomenon is better adapted for nitrogen fixation in low light than Anabaena. The response of both fixers to current and previous exposure may explain the dominance of Aphanizomenon in nitrogen-limited water bodies with low light penetration.