1. The upper seeds of Xanthium show constantly less catalase activity per unit of dry substance than the lower seeds. 2. Catalase activity is relatively stable in dry stored seeds, for several months. 3. Stored in imbibed condition in germinators below the temperature for germination, the catalase activity of both uppers and lowers increases immediately following exposure in the germinators. 4. In the field during the germination season, lower seeds increase in catalase activity, while the uppers lag behind them, showing practically no increase. 5. In the laboratory, soaking seeds on ice seems to depress activity slightly, but during subsequent germination, catalase activity increases pari passu with the germination. Upper seeds show much less increase than lowers, except when they actually germinate. 6. Long delay of germination while in the germinators leads first to increased catalase activity of uppers, followed by a decline, which at the end of several months leaves the seeds with no more activity than air dry seeds. The decreased catalase activity accompanies a decreased vigor of germination. 7. The catalase differences in Xanthium seeds are in harmony with other physiological differences, which cooperate to bring about delayed germination of upper seeds with intact coats.