The galls on Ulmus leaves formed by Tetraneura fusiformis enlarged gradually from the 1st week of gall development to the 3rd week and rapidly from the 3rd to the 4th week, although no further enlargement was, thereafter, observed. Offspring began to be produced by the fundatrix from the 4th week and reached a maximum in number in the 5th week. thereafter, the number of offspring did not change until the 7th week. Dry weight of leaves with or without galls increased similarly from the 2nd to the 7th week, though the dry weight of the latter was higher than that of the former throughout the season. Maximums of peroxidase and polyphenoloxidase activities in non-galled leaves were observed in the 1st week and the zero week, respectively, and then those enzymatic activities gradually fell, whereas those in the galls were at a maximum in the zero or fist week, then dropped rapidly and were almost lost at the 2nd (peroxidase) or 4th week (polyphenoloxidase). These observations suggest that the process of formation and utilisation of galls by insects is apparently divided into two broad physiological stages before and after the 4th week.