Lobaria oregana is an epiphytic macrolichen associated with old-growth Douglas-fir forests in the Pacific Northwest. Nitrogen fixation by this often-abundant cyanolichen provides an ecologically significant input of new N to the forest ecosystem. This study estimates annual N₂ fixation by L. oregana using a model based on physiological field measurements and laboratory experiments. Meteorological data from the Wind River Canopy Crane site and the H. J. Andrews Experimental Forest are used to calculate annual N₂ fixation rates, assuming that hydration and temperature are the two parameters controlling nitrogenase activity. At the crane site, estimated annual N₂ fixation is 1.5 kg ha⁻¹. In the H. J. Andrews Experimental Forest, L. oregana may fix 2.6–16.5 kg N₂ ha⁻¹ yr⁻¹ depending on its stand-level canopy biomass. The model's predictions are checked by using published growth rates and standing L. oregana biomass estimates to calculate independent values for annual N₂ fixation at each site.