Biofouling is ubiquitous in marine environment, and bacteria are among the first organisms to foul surfaces. They form biofilms which serve as focus for the attachment and growth of other organisms, such as invertebrates, sessile plants, and animals (Davis et al., 1989). Mature marine biofouling communities are complex, highly dynamic ecosystems (Fig.1) and once established are extremely difficult to eradicate (Holmstrom et al., 2002). For this reason the understanding of the mechanisms leading to marine bacterial attachment and its subsequent biofilm development are of great biological importance with obvious potential industrial outcomes. This development is conditioned by complex processes involving bacterial attachment to surfaces, growth, cell-to-cell communication, mobility and production of exoproducts constituting the biofilm matrix. Concerning attachment, the molecular strategies used by bacteria are diverse. They can employ pili, fimbria and a plethora of proteins regrouped under the term “adhesins” that recognize many different elements of the target (living or inert) surfaces (Pizarro-Cerda and Cossart, 2006; Fronzes et al., 2008). Although there is consistent data on human pathogenic bacteria attachment mechanisms, scarce information is available for marine bacteria. For this reason this project focused in the marine biofilm-forming bacterium Pseudoalteromonas sp. D41 (P. D41). This organism displays strong and competitive adhesion onto a wide variety of substrates, promoting subsequent biofilm development. Previous physicochemical studies in this strain related the high outer-shell protein content to its adhesion properties (Pradier et al., 2005; Leroy et al., 2008). For this reason, we attempted to unravel the molecular mechanisms responsible for these adhesive and competitive properties though a proteomic strategy, with particular attention to the outer membrane (OM) fraction.