It had been ascertained by the authors that nitrogen fertilizer accelerates root hair formation and growth in the lateral roots of rice plants. But the effects of nitrogen fertilizer on root hairs of crown roots have not been ascertained. The present investigation was carried out to study the morphogenetical effects of nitrogen fertilizer on the development of root hairs in crown roots, and the effect on the physiological longevity of the root hairs by means of ribonucleic acid (RNA) detection. The following results were obtained: Generally, the root hairs which developed on the thicker crown roots were more numerous per unit crown root length and longer in length than those of the thinner crown roots (Figs. 1, 2, 3 and 4). As regards the morpho-genetical effects of nitrogen fertilizer on the development of root hairs, it was found that root hair formation and growth in crown roots were accelerated when a high level of nitrogen fertilizer was given to the rice plants (Tables 3 and 5). It was also found that the percentage of root hairs containing RNA increased in proportion to the nitrogcn fertilizer amount given to the rice plants (Fig. 6). From these results it may be assumed that nitrogen fertilizer affects root hair formation and growth, and it also affects maturation of root hairs in crown roots.