Abstract:The feeding rhythm of the polychaeteNereis diversicolor(O.F. Müller) was studied during tidal cycles (spring and neap tide) at three sites in baie du Mont St-Michel and the Rance estuary in summer 1990. For most of the sites, the polychaetes were mainly feeding during spring tides when immersion occurred. Dietary activity decreased at emersion sites depending on low tide duration. During this period, the physical and chemical conditions in the mud flat sediments are not favourable to polychaetes’activities (high temperature, high desiccation, low oxygen content) and can cause mortality or migration of the population. Repletion indices from polychaetes of the Hirel sector (baie du Mont-St-Michel) showed a decrease of their dietary’s activity during the neap tide period even if the site was permanently immersed.