Barbatic acid, a lichen-derived depside, inhibited oxygen evolution in spinach thylakoid membrances. It also affected parameters of chlorophyll fluorescence, (Fm'-F)/Fm and Fv/Fm. Using specific donors and acceptors of electrons, we found two sites of inhibition in the PS II complex. The primary site, which is responsible for the inhibition of oxygen evolution in vivo, is at the reducing side of Q_A' probably at Q_B. The other site is at the oxidizing side of P680 but not in the oxygen evolving complex, suggesting Yz as the target. At both sites, irreversible binding of the depside to the targets seems to be responsible for the inhibition. Among the 8 lichen acids compared, barbatic acid was the most potent inhibitor for both the reducing side and oxidizing side. In contrast with spinach thylakoids, whole cells of Chlamydomonas reinbardtii, a green alga, showed stronger inhibition at the oxidizing side than reducing side. Synbiotic algae, Trebouxia impressa from Ramalina crassa and Trebouxia sp. From R. litoralis, showed resistance to the lichen acid even at 1 mM, suggesting the physiological role of lichen acids in allelopathy.