Three different effects (the primary, mitosis-free period and the secondary effects) of neutron bombardment on the root-tips of Vicia faba were described cytologically. The primary effect consists of advancement of the mitotic process from metaphase to telophase, retardation of the same mitotic process owing to the chromosome aberration and relative advancement of chromosome formation from prophase chromonemata. The former may chiefly be attributed to the effect on the cytosome and chromosome (dehydration) which resulted in an increase in the number of mitotic figures in the telophase, while the latter two are attributed to the effect an the chromosomes, namely fragmentation, clumping and hydration. These, moreover, result in an accumulation of mitotic figures in the metaphase and later stages.The mitosis-free period results from the delay in chromosome formation from the resting nuclei, induced after irradiation.The secondary effect, which reveals itself after the mitosis-free period, shows abnormalities appearing in the process of recovery of cells from the effect of the neutron bombardment. Fragmentation and fusion of the chromosomes and irregular mitoses were observed 96 hours after irradiation.The fragments formed were chiefly chromosome fragments and rarely chromatid fragments. The abnormal changes in somatic mitoses produced by the action of neutron rays may be comparable with those seen in the case of the desiccation treatment (cf. Wada 1936).These investigations were carried out according to the program of the Atomic Nucleus Sub-Committee of the Japan Society for the Promotion of Scientific Research to which we wish to express our gratitude. We are indebted to the Japan Wireless Telegraph Company for the electromagnet and other pieces of equipment used for the cyclotron, and to the Mitsui Hoonkwai Foundation, the Tokyo Electric Light Company, and Mr. G. Hattori, Director of K. Hattori Company, for financial aid. We acknowledge the kind assistances given by our colleges of the Nuclear Research Laboratory in connexion with the irradiation of samples.