Flocculation performances of cationized agaroses of different degrees of substitution in the range 0.04-0.77 are reported for the first time. The cationized agaroses were successfully synthesized by the reaction of agarose with 3-chloro-2-hydroxypropyltrimethylammonium chloride in alkaline medium. Two of the cationized agaroses with degrees of substitution of 0.19 and 0.58, presented colloid flocculation performances comparable to commercial cationic polyacrylamides, as determined from assays using kaolin suspensions as model systems. Zeta potential measurements for the cationized agarose that presented the best performance suggested that bridge formation, rather than charge neutralization, should be the main mechanism responsible for flocculation of kaolin particles. Cationized agaroses may constitute a new flocculating agent with promising properties for water treatment, especially if the source of the agarose is a seaweed species of easy availability. (C) 2011 Elsevier B.V. All rights reserved.