As a series of studies on the behaviour of Pine gallmidge (Theccdiplosis japonensis Uehida ei Inoaye), the season of the larval hibernation in winter as well as the season of the attack of adults on pine needles in the vicinity of Seoul were observed, and following results were obtained. 1. The larvae get out of the galls and fall into the ground for the hibernation commencing from the end of September and finishing by the end of January of the next year. 2. About 76% of the whole larvae fell down during the month of November. 3. The frequency and the duration of raining mainly influenced the larval falling into the ground but temperature, humidity and the quantity of rainfall was not likely influence upon it. 4. As many as 47,000 larvae were counted per 1.5 square meter of the ground under the crown of the damaged trees. 5. When pine needles were isolated by fine linen-net-bag to keep the needles from the oviposition of the adults of the insect, at six different season; -i.e. 30th, May, 6th, 10th, 15th, 25th, and 30th June, the percentages of the damage were 80%, 50%, 36%, 19%, 20% and 1% respectively, while the damage of the control was 91%.