The study in particular relates to a process for modifying surface of polymeric membrane and their biocidal activities. The modification process is based on absorption of 1, 3, phenylene diamine on the asymmetric polysulfone membrane and its diazo reaction. The azo compound (Bismarck brown) is characterized by LC-mass, uv-vis spectra. The incorporation of azo compound into polysulfone asymmetric membrane is proved by ATR-FTIR, SEM studies. The decrease in contact angle for modified sur-face proves the development of hydrophilic character. The modified membrane shows higher biocidal activity (for marine bacteria Vibrio sp.) compared to the virgin polysulfone membrane. The biofilm formation is inhibited for the modified membrane compared to virgin Polysulfone membrane. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115:371.0-3715,2010