The Hermelles bank is a bioherm built by sedentary polychaete annelids along the coast of the English Channel (Bay of Saint-Michel). Situated in the lower part of the intertidal zone, the bank forms a mass over 400 hectares in surface area. Compilation of a detailed map has permitted a precise analysis of the 'reef' constructions and the associated sediments (biogenic sands, fine sands, calcareous mud) and the proposal of a hypothesis concerning the genesis and evolution of the bank. The faunal association encountered is not characteristic of the bank; the bioherm is characterized by the favorable environmental conditions found there by the organisms. The hydrodynamic regime dominant in this part of the Bay of Saint-Michel is responsible for the establishment and development of the reef complex.