Antibacterial activities of six acid-soluble [two degrees of deacetylation (DD) × three viscosities] and two water-soluble chitosans (two DD with similar viscosities) were examined against eight gram-negative (Pseudomonas fluorescens, Proteus vulgaris, Erwinia carotovora, Serratia marcescens, Escherichia coli, Vibrio parahaemolyticus, V. vulnificus, and Salmonella Typhimurium) and six gram-positive bacteria (Listeria monocytogenes, Staphylococcus aureus, Bacillus subtilis, B. cereus, Lactobacillus curvatus, and L. plantarum). Antibacterial activities of chitosans differed depending on the chitosan types and bacteria tested. Water-soluble chitosans inhibited bacterial growth by one to eight log cycles at 0.1% concentration; however, the effects were much lesser than those observed with 0.05% acid-soluble chitosans. Minimum inhibitory concentration (MIC) values (0.03% to above 0.1%) of acid-soluble chitosans were much lower than those (0.05% to above 0.8%) of water-soluble chitosans. Based on MIC values, the acid-soluble chitosan with 99% DD and lower viscosity (17.9 mPa s) was most effective in inhibiting bacteria growth among eight chitosans tested.