Bacteria are flocculated with high molecular weight cationic synthetic flocculants and chitosan. High charge density polymers are the most effective of the synthetic flocculants. Only chitosan is effective in flocculating the E. coliand B. subtilis cultures in complex broth. The difference in effectiveness between the synthetic flocculants and chitosan for flocculating E. coli, B. subtifisand Z. mobilismay be attributed to hydrogen bonding between the polysaccharide flocculant and cell surface polymers in addition to electrostatic interactions, and, in complex media, complexation of synthetic polymers with anionic polyelectrolytes.