The effectiveness of the chlorination of seawater for an open intake reverse osmosis (RO) installation, by continuous chlorination (CC) or shock chlorination (SC), associated with two different pretreatment processes has been analyzed. Ultrafiltration membrane pretreatment and physicochemical pretreatment were evaluated, associated with CC (1 mg Cl-2/L) or with SC dosed fortnightly to obtain 1 mg Cl-2/L at the end of the pretreatment line, by measuring the presence of total aerobic bacteria (TAB) in pretreated water and the development of biofilm on the walls of the different tanks of the pretreatment lines. Bacteriological seawater quality was similar for both pretreatment systems but CC enabled better control of TAB than SC, according to its concentration or the number of positive samples. Attached bacteria were observed on the surface of the tanks of both assayed pretreatment lines but biofilm was only observed in the influent tank. The bacteriological quality of the seawater affected the degree of biofilm development, resulting in poorer biofilm control when SC was applied, regardless of the pretreatment used. Our results show that although chlorination hinders the development of biofilms on the RO membranes, with SC there is a greater risk of bacteria adhering to the surfaces of RO installations. (C) 2014 Elsevier B.V. All rights reserved.