The aim of thesis was to determine minimal inhibitory concentration (MIC) for didecyldimethylammonium chloride (DDAC) for Staphylococcus aureus, Escherichia coli, Salmonella enterica, Listeria monocytogenes and Bacillus cereus. Broth microdilution method with tryptic soybean broth (TSB) was used. By adding 2-p-iodophenyl-3-p-nitrophenyl-5-phenyl tetrazolium chloride (INT) reagent, MIC values were determined. At DDAC concentrations that correspond MIC values the number of surviving bacteria was determined by plate count method. The results confirmed our prediction that gram-negative bacteria of E. coli and S. enterica are more resistant to DDAC than gram-positive bacteria of Staph. aureus and L. monocytogenes. Spore forming bacteria of B. cereus were the most sensitive to DDAC among gram-positive bacteria. When using disinfectants in industry it is very important to follow the instructions for their use, including the anticipated concentrations. Inappropriate usage of disinfectants may have more negative than positive consequences and can lead to adaptation of bacteria or even a development of resistant strains. This can also lead to cross-adaptation or cross-resistance, thus lowering efficiency or inefficiency of other biocides, such as antibiotics.