The rice stem borer, Chilo suppressalis, is a major pest of the rice plant in Japan, and a large amout of insecticides is consumed every year for control of it. Recently, considerable interest attaches to the effect of chemical composition in host plants on development and fecundity of phytophagous insects, and in consequence, to the possibility of insect control through application of fertilizers. In view of some recent observations that the borer damage in the rice plants may be greatly affected by application of a large amount of fertilizers to paddy fields, the experiments were carried out to prove the effect of the nitrogenous fertilizer on the larval growth of the larvae.<br>A rice plant variety, Norin No.29, was cultured in nutrient solutions at two levels of nitrogen content, then larvae were allowed to feed on them. Two feeding experiments were performed, one of them was on the living rice plants in a greenhouse and the other was with the sterilized rice plant stem (borer zone) under aseptic conditions.<br>Rearing on the living rice plant. From the end of August to the beginning of September, the rice stem borer eggs just before hatching were placed on the rice leaf, and larvae hatched were allowed to feed on the living rice plants for 30 days. The results obtained are as follows:<br>Rearing with the sterilized rice stem. On 30th August, the rice plants growing in the nutrient solutions were reaped, and roots, leaves and ears were removed. 30g each of remaining stems (borer zone) chopped to 3 to 4cm in length was put in 300ml Erlenmeyer flasks with 6ml of water, and plugged with cotton, then sterilized at 18lbs per sq. inch for 15 minutes. With those rice stem diet the borer larvae were reared aseptically for 20 days at 28°C. The results are as follows:<br>Chemical composition of the rice plant stems. Chemical analyses on the rice plant stems samples on 30th August were performed to investigate the effect of nitrogen fertilizer. The results are given in the text in detail.<br>Under these conditions, significant superior growth of the larvae, as measured by their weights, is introduced by feeding on the rice plant cultured in high nitrogen solution. These rice plants contain a high level of nitro genous compounds and a rather small amount of carbohydrates to compare with the plants cultured in low nitrogen solution. The data from the previous (ISHII & HIRANO, 1958) and the present experiments clearly show a close correlation between chemical composition of the rice plant stems (borer zone) and the growth of the rice stem borer larvae.