Most studies of spore germination in seedless vascular plants have involved species that develop surficial, photosynthetic gametophytes following spore germination. However, several species, including Lycopodium clavatum, give rise to subterranean, nonphotosynthetic, mycorrhizal gametophytes and their spores germinate in the dark. Red light, like white light, inhibits the germination of these spores. Germination occurs after exposure to far-red light. The effects of far-red light are reversed by red light and those of red light are reversed by far-red light confirming the involvement of phytochrome. The active form of phytochrome, Pfr, inhibits germination in L. clavatum. It appears that this is a general phenomenon in seedless vascular plants with subterranean, mycorrhizal gametophytes because it is now known to occur in two species, L. clavatum and Ophioglossum crotalophoroides, from unrelated families. The photoinhibition of germination by white or red light insures that these spores germinate underground in nature providing improved chances of spores obtaining adequate soil moisture and mycorrhizal colonization of young gametophytes that are essential for continued development.