The effect of the released polysaccharide (RPS) of the cyanobacterium Aphanothece halophytica GR02 on the recovery of the alga by flocculation with ferric chloride was studied. With increasing RPS concentration in algal cultures from 0 to 68 mg L-1 the flocculation efficiency at the same dosage of ferric chloride decreased, and higher dosages of ferric chloride were required to attain the same flocculation efficiency. It is demonstrated that RPS could form complexes with ferrum during flocculation. In conclusion, RPS of A. halophytica GR02 had a significant inhibitory effect on flocculation of the alga with ferric chloride. The inhibitory mechanism of A. halophytica GR02 RPS allows the RPS to compete for ferrum by forming complexes with ferrum, thus leading to the consumption of ferrum in ferric chloride.