Listeria monocytogenes is a foodborne pathogen that causes the potentially life-threatening illness listeriosis. Previously, a few clones of L. monocytogenes persisting in a cold-smoked fish processing plant were isolated from the plant's products continuously. To evaluate the role of biofilms in the persistence of L. monocytogenes strains specific to this plant, the abilities of the persistent strain (PS) and transient strain (TS) of L. monocytogenes found in this plant to form biofilms were compared, as was resistance to the sanitizing effects of benzalkonium chloride (BC). The PS produced more biofilm than the TS in 48 h. The half-maximal effective concentration (EC50), the BC concentration at which the ATP bioluminescence of each bacterial strain decreased by 50 % relative to its maximum activity, was about 150-fold higher in the PS than in the TS. In contrast, when these values were measured in organisms in a planktonic state, the EC50 of the PS was only 2.2-fold higher than that of the TS. Extracellular polymeric substances (EPS) were extracted from biofilms, and the glucose content of these biofilms was determined with the phenol-sulfuric acid method to estimate the quantity of EPS. The total amount of EPS in the PS biofilm was higher than that in the TS biofilm. These findings suggest that the PS produces greater amounts of biofilm and EPS than the TS, which results in greater resistance of the PS to disinfectants. The persistence of the strain in the fish processing plant might be attributable to these properties.