We measured the timing and magnitude of nitrogenase activity and N2fixation by lupines colonizing early successional volcanic sites at Mount St. Helens. Nitrogenase activity (measured by acetylene reduction) in Lupinus lepidus growing at a pyroclastic site exhibited significant diurnal trends, with lowest ethylene evolution rates at night. Nitrogenase activity also followed seasonal trends, with high rates in June, very low levels in August, the dry warm part of the season, and a partial recovery of nitrogenase activity in September after precipitation resumed. A comparison of typical nitrogenase activities measured at several sites suggested that rates of N2fixation were highest in L. lepidus growing at disturbed low N sites. Adult lupine C and N composition also varied during the growing season, with trends correlated with seasonal patterns of nitrogenase activity. Seasonal N2fixation in L. lepidus and L. Latifolius was measured using15N isotope. Both species fixed °60% of their N during the first season of growth with some evidence of preferential allocation to aboveground biomass. N fixation by Lupinus lepidus individuals was ° 18.1 mg/g biomass or an average of 15.4 mg per plant, while L. latifolius fixed an average of 16.3 mg/g biomass, equivalent to 22.9 mg per plant. Average net C fixation during the same period was 355 and 589 mg per plant for L. lepidus and L. latifolius, respectively. Despite these rates, the current distribution of L. lepidus into a few, small patches that occupy<1% of the surface area indicates that annual N inputs by lupines are<0.05 kg/ha and thus probably not the primary source of N input into developing Mount St. Helens pyroclastic sites except at a local scale.