Growth characteristics of hydrilla (Hydrilla verticillata (L.F. Royle) grown under equal photon flux density at various portions of the visible spectrum were compared. Stem elongation was most pronounced under green light, whereas shorter and highly-branching plants were produced under red light. Dry weight yields were higher under red and blue light while green and white light produced the lowest yields. The total chlorophyll content was not affected by these light quality regimes. However, the chlorophyll a:b ratio was lower in plants grown under green light than in other treatments. A similar decrease in the a:b ratio was observed in hydrilla plants sampled at increasing water depths under natural conditions. In vallisperia (Vallisneria neotropicalis Marie-Vict.), however, no change in the a:b ratio was observed under similar natural conditions. The changes in chlorophyll composition in favor of chlorophyll b, is discussed in terms of a possible adaptation by hydrilla to the spectral change of solar radiaion in deeper waters.