Evidence is presented that a naturally occurring hydrilla‐growth inhibitor also inhibits the germination of lettuce seeds. The effect is concentration dependent, and the development of both hypocotyledons and dicotyledons is retarded, though no morphological changes are noted in comparison with control samples. In addition, a photodynamic action is involved: the number of seeds developing cotyledons and hypocotyledons in a given time is significantly reduced in the samples exposed to light and the growth inhibitor in comparison to control samples exposed to inhibitor, but maintained in the dark. From experiments with sodium azide, it seems probable that the observed photodynamic action is mediated by singlet oxygen (O21Δg).