Research on community structure and dynamics has been carried out in a Cystoseira zosteroides community from Tossa de Mar (Northwestern Mediterranean). The Mediterranean endemic Cystoseira zosteroides is a rheophilous and sciaphilic species that occours in reduced areas from the circalittoral zone, always below 15 m depth. Cystoseira zosteroides is the dominant species in a widespread association previously described as Cystoseiretum zosteroidis by means of phytosociological methods. Other Phaeophyceae such as Dictyota dichotoma, Halopteris filicina and Cladostephus hirsutus are seasonally abundant in the community. Encrusting algae (e.g. Lithophyllum incrustans, Mesophyllum lichenoides, Peyssonnelia rosa-marina) cover the entire substrate and represent a high percentage of the total biomass. Algal biomass ranges between 1250 and 1900 g dwt m^-2 (including calcareous algae) and percentage coverage ranges between 130 and 360 %. Species richness and species diversity are high in comparison with other Mediterranean Cystoseira communities; these features have been related to the coexistence of photophilic and sciaphilic algae .within the community. Areas of 900 cm lang=EN-GB style='font-size:5.5pt'>2 are recommended in order to collect representative samples of the community. There is a great structural and functional variation all along the year as it occurs in other Mediterranean phytobenthic communities. The production phase occurs in spring, while the diversification phase occurs in late summer and autumn. Growth of Cystoseira zosteroides and other conspicuous algae seems to be mainly limited by light availability in winter, although nutrient availability probably limits growth in summer. A spring growth of primary laterals partially supported by photosynthetate translocation from the tophules placed in the bases of such branches is suggested. Annual production of Cystoseira zosteroides has been estimated to be 24 g C m^-2 y^-l and overall community production amounts to 81 g C m^-2 y^-l. These production values are similar to those obtained in other light-limited phytobenthic communities.