The diazotrophic cyanobacterium Trichodesmium is a significant contributor to marine nitrogen and carbon cycles and has been incorporated in biogeochemical ocean circulation models. To date, parameterization of light as a controlling factor for nitrogen fixation has been based on field observations, where factors other than light also affect Trichodesmium physiology. Here we present data on light-dependent (15 to 1100 μmol quanta m-2 s-1) diazotrophic growth from controlled laboratory experiments and their implications for modeling approaches. We supply a simple empirical model to describe nitrogen fixation by Trichodesmium in batch cultures. Diazotrophic growth of axenic Trichodesmium IMS-101 was light saturated at 180 μmol quanta m-2 s-1 and did not vary significantly at higher photon irradiances up to 1100 μmol quanta m-2 s-1 (μcarbon based ≈ 0.26 d-1). Chlorophyll a (chl a) normalized N2 fixation rates were significantly affected by light intensity during mid-exponential growth (0.74 to 4.45 mol N fixed mol chl a -1 h-1) over the range of photon irradiances tested. In contrast, nitrogen fixation rates normalized to the cellular carbon content were relatively unaffected by light intensity (0.42 to 0.59, averaging 0.5 mmol N mol particulate organic carbon [POC]-1 h-1). Trichodesmium carbon biomass can be used to estimate the nitrogen input by this diazotroph into the ocean; the maximum input rate is 350 nmol N fixed l-1 h -1. © Inter-Research 2008.