The observations on the seasonal abundance of insect pests attacking the young branches and leaves of citrus tree were carried out since the spring of 1962 at the citrus orchard of Omura City, Nagasaki Prefecture. Thirty-three Satsuma orange (Unshu Mikan) trees of six or seven years old were used for this study. In this orchard, the pesticides were not sprayed since the beginning of the study. The most injurious insect pests of the young branches observed in the orchard were composed of the undermentioned four species. (On the contrary, old branches were attacked by some species of scales, white flies and mites). Citrus leaf miner, Phyllocnistis citrella STAINTON (larvae) Citrus leaf roller, Agonopterix culcitella HERRICH-SCHAFFER (larvae) Citrus swallowtail, Papilio xuthus L. (larvae) Citrus aphid, Toxoptera citricidus (KIRKALDY) (nymphs and adults) All of the larvae of citrus leaf roller in the rolled leaves, larvae and pupae of citrus leaf miner in mines, eggs, larvae and pupae of citrus swallowtail and the nymphs, adult of citrus aphid were counted once a week. At the same time, the number of young leaves available for the food of these insects were counted. The maximum number (summation of 33 trees) of the citrus leaf miner is 3830 individuals, citrus leaf roller 586,larvae of citrus swallowtail 15,and the citrus aphid (nymphs) exceeded 15000. In this district, the citrus trees normally shoot three times a year. Sometimes, only one or two times of shooting takes place in a year according to the age or other conditions of the trees. The leaves are most abundant on the spring shoot. In 1962,two times of shooting viz. the spring shoot and the summer shoot were seen in the orchard. Citrus leaf roller and citrus swallowtail showed two peaks of population density. But, the citrus leaf miner and the citrus aphid were most abundant at the time of the summer shoot only (Fig. 3 and 5). In 1963,three times of shooting of citrus tree takes place in this orchard. Citrus leaf roller and citrus swallowtail showed three fluctuations in this year in the orchard. Citrus leaf miner and aphid were most abundant upon the summer and autumn shoot (Figs. 4 and 6). From the observations, it seems that the seasonal abundance of the population of the citrus leaf roller and citrus swallowtail synchronizing with the shooting (it means the seasonal abundance of available food of each species) of citrus trees. But the population size of the citrus leaf miner and the citrus aphid grow up as the summer and autumn shooting. The former two species has three (or sometimes four) generations in one year. The seasonal abundance (which are determined by the time of shooting) of them has the synchronization with the physiological rhythms of citrus trees. And the later two species has ten or more generations in a year. The seasonal abundance of these species may be determined by the time and amount of the summer and autumn shoot of the young leaves.