A quantitative study of the fixation of nitrogen and of its transfer from the nodules to the remainder of the plant has been made in Alnus plants during their first season of growth. Fixation per plant reached a maximum in late August but fell rapidly with the onset of autumn, while fixation per unit dry weight of nodule tissues was greatest in young nodules and was of the same order as in nodulated legumes. Throughout the growth season there was a steady transfer from the nodules of some 90 per cent, of the nitrogen fixed. The rate of fixation relative to the growth of the endophyte is much higher than in free-living nitrogen-fixing organisms and is clearly governed by the nitrogen requirements of the entire symbiotic system. These findings are compatible with the view that the fixation process is extracellular to the endophyte.