The epibiotic iron-encrusted biofilms occurring on the shell of the bivalve Montacuta ferruginosa were examined over an annual cycle in order to determine possible variation of the iron-encrustation. Among the 379 examined specimens, well-coated bivalves always predominate. However, it was found that about 50% of the small-sized bivalves were uncoated or weakly coated during the summer months. This result can be explained by the life history of the bivalve and by the physico-chemical conditions in the sediments.