Biofilm extracellular polysaccharides are compounds secreted by microorganisms into the surrounding environment and which are important for surface attachment and maintaining structural integrity within biofilms. They have been suggested to be metabolically costly to produce, and because they are secreted, to act as co-operative shared resources within biofilm communities. These assumptions have not been experimentally well-examined. Here we empirically test the cooperative nature of the PSL polysaccharide, which is crucial for the formation of biofilms in Pseudomonas aeruginosa. We show that: (1) PSL provides population level benefits in biofilms, for both growth and antibiotic tolerance; (2) the benefits of PSL production are social and are shared with other cells; (3) the benefits of PSL production appear to be preferentially directed towards cells which produce PSL; (4) cells which do not produce PSL are unable to successfully exploit cells which produce PSL. Taken together, this suggests that PSL is a social but non-exploitable trait, and that growth within biofilms selects for PSL-producing strains.