Kaempferol, a naturally occurring flavonol, inhibited coupled electron transport and both cyclic and noncyclic photophosphorylation in isolated pea (Pisum sativum) chloroplasts. Over a concentration range which gave marked inhibition of ATP synthesis, there was no effect on basal or uncoupled electron flow or light-induced proton accumulation by isolated thylakoids. It is suggested that kaempferol acts as an energy transfer inhibitor.