Several bacteria resistant to benzalkonium chloride (BC, alkyldimethylbenzylammonium chloride) were isolated. A representative strain, tentatively identified as Enterobacter cloacae, grew even in the presence of 10% or more of BC.The bacterial cells grown in the presence of BC showed a high content of cellular lipid and formed big capsules, indicating that BC induced the bacterium to change its phenotype. During culture, the BC added to the medium decreased with the growth of the bacterium, and inversively the cell-bound BC increased. The BC-resistance of the bacterium, however, was greatly affected by the kind of nitrogen source, temperature and pH value of the medium. The presence of ammonium ion resulted in the loss of the BC-resistibility of the bacterium.