Abstract—Inhibitory effect of chitosan (molecular mass 28 kDa, deacetylation 94%) and of its quaternized derivative with 60% substitution on bacteria of various taxonomic groups was investigated. Bacteria differing in the cell wall surface characteristics and affinity to dyes were found to differ in theri sensitivity to chitosan. Correlation dependencies between antibacterial activity of quaternized chitosan and the surface characteristics of bacterial cell walls were revealed.