Seventy-nine strains of bioflocculant-producing bacteria were isolated from 3 activated sludge samples. Among them, strain MYC was found to have the highest and stable flocculating rate for both kaolin clay suspension and oil-field produced water. The bacterial strain was identified as Klebsiella sp. MYC according to its morphological and biochemical characteristics and 16S rDNA sequence. The optimal medium for bioflocculant production by this bacterial strain was composed of cane sugar 20 g L -1, KH2PO4 2 g L -1, K2HPO4 5 g L -1, (NH4)2SO4 0.2 g [KG-*9]L -1, urea 0.5 g L -1 and yeast extract 0.5 g L -1, the initial pH being 5.5. When the suspension of kaolin clay was treated with 0.5% of Klebsiella sp. MYC culture broth, the flocculating rate reached more than 90.0% in the presence of 500 mg L -1 CaCl2, while the flocculating rate for oil-field produced water was near 80[KG-*3].0% in a pH range of 7.0-9.0 with the separation of oil and suspended particles from the oil-field produced water under similar conditions. The environment-friendly nature of the bioflocculant and high flocculating rate of the strain make the bioflocculant produced by Klebsiella sp. MYC an attractive bioflocculant in oil-field produced water treatment. Seventy-nine strains of bioflocculant-producing bacteria were isolated from 3 activated sludge samples. Among them, strain MYC was found to have the highest and stable flocculating rate for both kaolin clay suspension and oil-field produced water. The bacterial strain was identified as Klebsiella sp. MYC according to its morphological and biochemical characteristics and 16S rDNA sequence. The optimal medium for bioflocculant production by this bacterial strain was composed of cane sugar 20 g L -1, KH 2 PO 4 2 g L -1, K 2 HPO 4 5 g L -1 (NH4) 2SO4 0.2 g [KG- * 9] L -1, urea 0.5 g L -1 and yeast extract 0.5 g L -1, the initial pH being 5.5. When the suspension of kaolin clay was treated with 0.5% of Klebsiella sp. MYC culture broth, the flocculating rate reached more than 90.0% in the presence of 500 mg L -1 CaCl 2, while the flocculating rate for oil-field produced water was near 80 [KG- * 3] .0% in a pH range of 7.0-9.0 with the separation of oil and suspended particles from the oil-field produced w The environment-friendly nature of the bioflocculant and high flocculating rate of the strain make the bioflocculant produced by Klebsiella sp. MYC an attractive bioflocculant in oil-field produced water treatment.