This study isolated extracellular polysaccharides (EPS) as a powder material from cyanobacterial blooms and the powdered EPS was used to trigger colony formation of dispersed unicellular M. aeruginosa by controlling EPS concentration in culture medium. The effect of Ca2+ ions on the colony formation of M. aeruginosa was also investigated, then the interaction between EPS and Ca2+ ions on colony formation was discussed. The results showed that the addition of the powdered EPS into the medium did not cause morphological changes of M. aeruginosa, suggesting that EPS alone would not induce the colony formation of M. aeruginosa. On the other hand, a high concentration of calcium ions (1000 mg/l) caused colony formation. When EPS and Ca2+ ions in the culture medium were adjusted to 200 and 1000 mg/l, respectively, the colony density, the average cell number per colony and the particle size of M. aeruginosa showed ca. 1.7-2.0 times greater values than those in the Ca2+ added medium. Calcium ion contributed to the aggregation of M. aeruginosa via crosslinked reaction with negatively charged M. aeruginosa cells, and the addition of EPS possessing negatively charged functional groups such as carboxy groups could enhance the reaction, promoting the crosslinked reaction between EPS and Ca2+ ions.