Isotopic studies are presented on six of the eight recognized non-legume nodule-forming genera of Angiosperms. Evidence has been obtained of the occurrence of fixation of atmospheric nitrogen in the root nodules of Casuarina, Ceanothus, and Shepherdia, now examined isotopically for the first time. In further studies of the nitrogen-fixing nodules of Hippophaē and Alnus it is shown that the fixation continues for a longer period after detachment from the plant than is the case with legume nodules, and that the enrichment in 13N which they (and also detached Casuarina nodules) finally achieve after exposure to excess of the free isotope considerably exceeds that shown by detached legume nodules. Fixation in detached Myrica nodules was not clearly affected by reduction in oxygen supplied until the proportion of oxygen was less than 5 per cent.