Effects of light and gibberellic acid (GA(3)) application on the germination of Dioscorea tokoro Makino and Dioscorea tenuipes Franch. et Savat. were observed. For complete germination, seeds of both species required prechilling in moist condition before incubation at a higher temperature. Red light irradiation during the incubation after the prechilling promoted germination; blue, green, or far red light markedly inhibited the germination of both species.Application of GA(3) induced complicated changes in the germination of both species in relation to light quality. In the germination of D. tokoro, GA(3) inhibited in the dark and red; however, it promoted germination in blue and far red light. GA(3) promoted germination of D. tenuipes in the dark and in blue, green, or far red light. These phenomena are explainable by assuming two counteractive reactions (germination-promoting and germination-inhibiting) which are both activated by applied GA(3).