Extracellular polysaccharides are compounds secreted by microorganisms into the surrounding environment and which are important for surface attachment and maintaining structural integrity within biofilms. The social nature of many extracellular polysaccharides remains unclear, and it has been suggested that they could function as either co-operative public goods, or as traits that provide a competitive advantage. Here we empirically test the co-operative nature of the PSL polysaccharide, which is crucial for the formation of biofilms in Pseudomonas aeruginosa. We show that: (1) PSL is not metabolically costly to produce; (2) PSL provides population level benefits in biofilms, for both growth and antibiotic tolerance; (3) the benefits of PSL production are social and are shared with other cells; (4) the benefits of PSL production appear to be preferentially directed towards cells which produce PSL; (5) 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 relatively non-exploitable trait, and that growth within biofilms selects for PSL-producing strains, even when multiple strains can interact (low relatedness).