Biofilms are microbial communities that are characterized by the presence of a viscoelastic extracellular polymeric substance (EPS). Studies have shown that polysaccharides, along with proteins and DNA, are a major constituent of the EPS and play a dominant role in mediating its microstructure and rheological properties. Here, we investigate the possibility of entanglements and associative complexes in solutions of extracellular polysaccharide intercellular adhesin (PIA) extracted from Staphylococcus epidermidis biofilms. We report that the weight average molar mass and radius of gyration of PIA isolates are 2.01 × 105 ± 1200 g/mol and 29.2 ± 1.2 nm, respectively. The coil overlap concentration, c*, was thus determined to be (32 ± 4) × 10-4 g/mL. Measurements of the in situ concentration of PIA (cPIA,biofilm) was found to be (10 ± 2) × 10-4 g/mL. Thus, cPIA,biofilm < c* and the amount of PIA in the biofilm is too low to cause polymer chain entanglements. In the pH range 3.0-5.5, PIA was found to both self-associate and to form complexes with bovine serum albumin (BSA). By static light scattering, both self-association and complex formation with 0.5% (w/v) BSA were found to occur at PIA concentrations of 0.30 × 10-4 g/mL and greater, which is about 30 times lower than the measured cPIA,biofilm. These results suggest that the microscopic origin of EPS viscoelasticity is unlikely to be due to polysaccharide entanglements. Furthermore, the onset of self-association and protein complexation of PIA occurs at concentrations far lower than the native PIA concentration in biofilms. This finding therefore suggests a critical role for those two association mechanisms in mediating biofilm viscoelasticity. © 2013 American Chemical Society.