The main goal of this work was to study the dynamics and biochemical composition of ExtraCellular Polysaccharides (ECPS), a fraction of the Extracellular Polymeric Substances (EPS) produced during the development of a microphytobenthic biofilm in a European intertidal mudflat (Marennes-Oléron Bay, France) during winter. Microphytobenthic biomass was surveyed during four consecutive emersion periods to confirm the biofilm growth. Bacteria abundance was also checked considering the importance of heterotrophic bacteria observed by various authors in the dynamics of EPS. Various colorimetric assays, coupled to biochemical chromatographic analysis were used to characterize the three main fractions of extracted EPS: colloidal, bound and residual. The monosaccharide distribution of colloidal ECPS highlighted their role of carbon source for bacteria (> 50 % of glucose) even if no increase of colloidal carbohydrate amounts was observed during the tidal exposure. Bound ECPS were composed of deoxy or specific sugars (30 % rhamnose) and uronic acids (18 % galacturonic acid). Their levels and dynamics could be correlated to the development of the microphytobenthic biofilm, enhancing the stabilization of the sediment or increasing binding forces accordingly. Residual fractions, containing refractory bound ECPS and other internal polymeric substances, were composed of various carbohydrates. The high ratio of glucose in these fractions (18 to 43 %) was interesting as it was once attributed to colloidal sugars due to poor extraction procedures. Finally, the presence of inositol (15 %) was significant since no author has highlighted it before, knowing that inositol is a major growth factor for heterotrophic bacteria.