Summary The action of the energy transfer inhibitor tentoxin on the delayed fluorescence of chloroplast suspensions of the tentoxin-sensitive plant spinach as well as of suspensions of the green alga Chlorella and the blue-green alga Anacystis was investigated under different conditions of electron transport. Tentoxin increased the delayed fluorescence in all three species, indicating an unspecific effect, which cannot account for the observed sensitivity of some plants to this toxin. A clear distinction between delayed fluorescence of control and tentoxin-treated chloroplasts was achieved only under phosphorylating conditions (i.e., in the presence of ADP and inorganic phosphate). The curves for induction of delayed fluorescence suggest a decrease of the proton conductance of the thylakoid membrane after tentoxin treatment. A concomitant increase of the potassium conductance did not occur. The tentoxin effect on delayed fluorescence was qualitatively the same as that of the ATPase inhibitor DCCD. Consequently it was explained by the known inhibition of photo phosphorylation by the phytotoxin. The derivative dihydrotentoxin enhanced delayed fluorescence only slightly.