We investigated both the current status of N2 fixation in western Washington forests, and the potential effects of acid rain on this vital process. Even the low concentrations of SO2 presently found in the Northwest are thought to have an adverse effect on N2 fixation by limiting the distribution of the epiphytic N2-fixing lichen, Lobaria pulmonaria, which is found mainly in deciduous forests. A close relative, L. oregana, was found to be the major N2 fixer in old-growth coniferous forests. It fixes less N2 following exposure to H2SO4 of pH 4 or less. A more serious threat to N2 fixation than acid rain is the practice of deliberately suppressing red alder to keep it from competing with Douglas fir. Also, L. oregana is a late successional species and does not develop in forests where short cutting cycles are practiced.