Diatom biofilms growing at the surface of the intertidal mudflat of Marennes Oléron, France, were incubated for 48 h in the laboratory under simulated conditions of high- and low tide (immersed and emersed in seawater) and day and night (illuminated or dark conditions). The biofilms were subsequently sampled using the cryolander technique, without disturbing the structure. The samples were kept in liquid nitrogen until they were transferred to the cooled stage of a field-emission cryo-scanning electron microscope, which was used to study the structural relationships between the sediment particles, the diatoms and the different types of extracellular polymeric substances (EPS) produced by these organisms. The diatoms were most abundant at the sediment surface when incubated in the light under emersed conditions. In the dark or when immersed, the diatoms migrated into the sediment. In the light, the diatoms were coated with EPS, while this was not the case when incubated in the dark. When immersed, the sediment surface appeared smooth as the result of the deposition of mud. Under emersed conditions, the coarser silt grains were prominently present. These grains were wrapped with organic matter and bound together through threads of EPS. This was the case both in light and in dark incubated sediment. It is proposed that this latter type of EPS contributes to the increased erosion threshold of intertidal mudflats colonized by biofilms of diatoms.