A quality index Ics (= index of community structure) has been developed as a single numeric descriptor to assess the structural state of macroalgal communities and to evaluate their relative development on rocky shores. Coverage of seaweed species were sampled on the field and treated by taxonomic groups, size classes and structural - functional groups (SFG), giving sub-indices It (= taxonomic index), Is (= stratification index) and Io (= organization index), respectively. Six macroalgal communities corresponding to canopies (belts) distributed vertically on the shore have been investigated for several years in 14 sites of Brittany (France). Communities developing at high topographic levels could be distinguished from those occurring at mid-tide to low levels by la values. Communities were globally stable over the sampling period, with few geographical discrepancies. Seasonal or sub-community variations could be tracked by sub-index values. The value of Ics to demonstrate the potential evolution of seaweed communities is discussed by comparison to other quality indices and in the perspective of ecological perturbations.