Chromosomes in the root nodule cells and root tip cells of three species of Alnus, A. hirsuta, A. firma and A. pendula were observed. It was found in each species that the root nodule cells possessed diploid chromosomes which closely resembled in form and size to those of the root tip cells. These observations are comparable with those of the diploid nodules of leguminous plants. Somatic chromosome numbers of the present three species were determined for the first time to be A. pendula 2n=28, A. hirsuta 2n=56 and A. firma 2n=112. It was found that A. hirsuta of Japan was a tetraploid form of this species.