The effects of temperature and daylength were studied in the germination of American pondweed (Potamogeton nodosusPoir.) tubers. Germination was examined at temperatures ranging from 5 to 30°C in conjunction with light treatments of short and long daylength and continuous darkness. Greatest percentage of tuber germination occurred between 15 and 20°C. Within this temperature range, daylength did not significantly affect germination. Outside this temperature range, however, the absence of light inhibited germination. From these results, we conclude that temperature was the most important factor controlling germination, with an optimal range of 15 to 20°C. Daylength (14:10 versus 10:14) had no effect on germination, but the absence of light suppressed germination at 10, 25, and 30°C. The results were further supported by similar data obtained from a pond study ofP. nodosus.