Environmental pollution by antibiotics poses a potential ecological risk to aquatic photosynthetic organisms. In the present study, toxic effects of erythromycin on PSI and PSII were investigated in cyanobacteria culture medium of Microcystis aeruginosa. The activity and electron transport of both photosystems were affected by erythromycin in a concentrationdependent manner. The quantum yield of PSII (Y_II) was reduced at 0.1 mg L^-1 of erythromycin, while the quantum yield of PSI (Y_I) significantly decreased at concentration of 5-25 mg L^-1. The decline of Y_II was accompanied by an increase of nonregulated energy dissipation (Y_NO). At 10 mg L^-1 of erythromycin, Y_II decreased by 55%, while Y_NO increased by 18%. The decrease of Y_I induced by erythromycin was caused by donor-side limitation of PSI (Y_ND). Y_ND was markedly enhanced with elevated erythromycin concentration. At 10 mg L^-1 of erythromycin, Y_I and Y_NA (PSI acceptor-side limitation) decreased by 8 and 82%, respectively, while Y_ND rose by 314%. The quantum yield of cyclic electron flow increased significantly at 0.1-1 mg L^-1 of erythromycin; it decreased but remained higher than that of the control at 5-25 mg L^-1 of erythromycin. The contribution of cyclic electron flow to Y_I, and to linear electron flow rose significantly with the increasing erythromycin concentration. The maximum values of electron transport rates in PSII and PSI decreased by 71 and 24.3%, respectively, at 25 mg L^-1 of erythromycin. Compared with the untreated control, the light saturation of PSII and PSI decreased significantly with increasing erythromycin concentration. We showed that concentrations of erythromycin ≥ 5 mg L^-1 could exert acute toxicity to cyanobacteria, whereas the chronic toxicity caused by concentrations of ng or μg L^-1 needs further research.