It was focused on the effect of different sludge concentrations on the performances of an algal-activated sludge symbiotic system in terms of wastewater treatment,algal-activated sludge characte ristics and community structure.The results showed that the highest nutrient removal efficiencies were obtained in the reactor R2 with soluble chemical oxygen demand (sCOD),ammonia nitrogen (NH4+-N) and phosphate (PO34--P) removal efficiencies of (90.6 ± 2.3)％,(97.69 ± 2.6)％ and (83.81 ± 2.3)％,respectively.Further investigation exhibited that sludge concentration has a great effect on the dissolved oxygen (DO) concentration,the pH,the growth of algae and the extracellular polymeric substance (EPS) production,which resulted in influencing the settleability and the performance of symbiotic system.The denaturing gradient gel electrophoresis (DGGE) analysis demonstrated that the sludge concentration had a selective power for particular members of algae.Meantine,the stimulated algal population would selectively excite the members of bacteria benefited for the formation of algal-bacterial consortia.The variation of microbial compositions,which was influenced by the different sludge concentrations,might be ultimately responsible for the different treatment performances.