Extracellular ATP (eATP) can function as a signalling molecule to regulate a wide range of cellular processes. This present work investigated the role of eATP in mediating the change of PSII (photosystem II) photochemistry of the tissues of Phaseolus vulgaris leaves infected with Xanthomonas campestris pv. phaseoli (Xcp). Infection of the leaves with Xcp caused a significant decrease in the the PSII maximal photochemical efficiency (Fv/Fm), the maximum quantum efficiency of PSII photochemistry at illumination (Fv′/Fm′), the PSII operating efficiency (ΦPSII), the rate of non-cyclic electron transport through PSII (ETR), photochemical quenching (qP), and eATP level in the tissues of the infected leaves. At the same time, the levels of non-photochemical quenching (qN) and the quantum yield of regulated energy dissipation of PSII (Y(NPQ)) were significantly increased. Application of exogenous ATP at 0·2 mm to uninfected leaves had no significant effect on any of the chlorophyll fluorescence parameters being measured. However, in the tissues of infected leaves, the application of exogenous ATP alleviated the decreases of the Fv/Fm, Fv′/Fm′, ΦPSII, ETR, qP and eATP level, and also abolished the increases of qN and Y(NPQ). These results suggest that the change of PSII photochemistry by pathogen infection could be mediated by eATP.