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The effects of three types of antibiotics (erythromycin,ciprofloxacin and sulfamethoxazole) on the photosynthesis of freshwater algae,Selenastrum capricutum Printz,were investigated by determining the growth rate,chloroplast pigments content,seven main precursors (including δ-aminolevulinic acid,porphobilinogen,uroporphyrinogen Ⅲ,coproporphyrinogen Ⅲ,protoporphyrin Ⅸ,Mg-proporphyrin Ⅸ and protochlorophyllide),and photosynthetic rate during chlorophyll biosynthesis.The antibiotics significantly decreased the growth rate,chlorophyll content,and photosynthetic rate.Erythromycin induced a decreasing effect at a concentration of 0.06 mg/L,while ciprofloxacin and sulfamethoxazole achieved the same results at concentrations higher than 1.5 mg/L.Only erythromycin significantly inhibited chlorophyll biosynthesis,which indicated that it was considerably more toxic to S.capricutum than ciprofloxacin and sulfamethoxazole,and may pose a high potential risk to aquatic ecosystems.