This study examined whether gravity influences the growth direction of dark-grown gametophytes of the fern Ceratopteris richardii. Analyses of directional growth of gametophytes in response to gravitropic stimulation demonstrated that gametophytes showed negative gravitropism. Dark-grown gametophytes of dkg1 her1 mutants, which germinate in complete darkness, displayed a more distinct negative gravitropism. Unlike her1 spores, dkg1 her1 spores do not require light irradiation to induce spore germination. Therefore, light irradiation on her1 spores was possibly inhibiting the negative gravitropism of her1 gametophytes. In the present study, prolonged white-light irradiation on her1 spores inhibited negative gravitropism in the gametophytes. Light irradiation on spores therefore affects the later negative gravitropism of dark-grown gametophytes.