In this study, N-hexanoyl-homoserine lactone (C6-HSL), a member of the N-acyl-homoserine lactone class of microbial quorum sensing (QS) signaling molecules, was used to improve microbial flocculant production. After exogenous C6-HSL was added, exopolysaccharide concentration of microbial flocculants was improved by 1.6-fold and flocculation rate of microbial flocculants was increased by 10 %. Fermentation conditions with added C6-HSL were further optimized through response surface methodology. The obtained optimal fermentation conditions were as follows: added C6-HSL concentration of 0.45 mu M, fermentation temperature of 30.4 A degrees C, and initial fermentation pH of 7.25. Under these optimal fermentation conditions, the resulting exopolysaccharide concentration was improved by 1.75-fold and flocculation rate was increased by 10 % compared with that of the control group. The yield of microbial flocculants was also improved by 1.75-fold. Results demonstrated that the existence of QS system in Agrobacterium tumefaciens strain F2 played the important roles in the microbial flocculant production.