A series of experiments to determine the effect of light quality and light intensity on sexual expression inEquisetumgametophytes showed that one species,E. arvense, is strongly affected by light intensity; another species,E. hyemale, is strongly affected by light quality; and a third species,E. fluviatile, is relatively unaffected by either light quality or light intensity. Supplementation of mineral media with sucrose indicates that this light effect is not photosynthetic, and in fact suggests that sucrose itself has a morphogenetic effect on sexual determination inEquisetumgametophytes.