Field and laboratory studies were carried out to analyse the development of biofilms generated on various metallic and non-metallic coupons immersed in coastal waters of Bombay. It is observed that the nature of the biofilm formed not only varies from substratum to substratum but is also influenced by the quality of water, flow conditions and the biotic status of the seawater. The biofilms depending upon the degree of entrapment of the inorganic detritus have varying weights. The biofilms formed on metallic surfaces when compared with perspex surface, are both qualitatively and quantitatively richer. Cupro-nickel, despite its toxicity, supports denser film than the one developed on inert perspex surface. The metallic coupons, both inert and toxic, support thicker films in polluted waters than in clean waters.