Research on leaf-epiphyte community structure and dynamics was carried out in Posidonia oceanica beds from Tossa de Mar (Western Mediterranean) . Three sampling stations w here chosen at 2, 9 and 23 m depth . Seasonal community structu re was studiea along the year 1982 in the 9 m station. Species composition, species covering , species richness , specific distribution , homogeneity (Kulczynski 's simi larity index) , species diversity, pattern diversity and the qualitative and quantitative minimal areas were obtained from each sample. These parameters were estimated from the species/ number of shoots curves, the diversity (Shannon index ) / number of shoots curves and the similarity/ number of shoots curves as indicated by BALLESTEROS (1986). Structura l changes in the epiphyte community have been observed; there is an increase in species richness and a decrease in the specific distribution and pattern-diversity from autumn to summer. Two different phases can be distinguished : a settling phase, where different shoots have a rather different species composition, that begins in October, just after the fall of senescent Posidonia leaves; and a cana li zation phase, where there is a prog ressive concurrence in the qualitative and quantitative composition of leaf-epiphytes between shoots. Comparison with other Medi t erranean algal communiti es shows t he miniaturization, the great homogeneity and the scanty structure of the Posidonia leaves ep iphytic community. Dynamics were studied in the three sampling stati ons. Species composition of Posidonia leaves is well cllaracterized by the abundance of Fosliella species and the encrusting pllaeophycean Myrionema magnusii. Other species such as Giraudia sphacelarioides, Ca stagnea irregularis, C. cylindrica, Giffordia mitchelliae, Myriactula gracilis, Ectocarpus siliculosus V . confervoides and Feldmannia globifera llave a spring maxima in tlle shallow stations. Zooepiphytes are more abundant, in relative terms , in the deepest station (basically the hydrozoan Sertularia perpusilla and bryozoans Electra posidoniae and Fenestrulina joannae) . Mean phytoepiphyte biomass vari es from 12 to 495 mg dw/ shoot (11 -470 9 dw/ m2) in tlle slla llow stations and 4 to 335 mg dw/ shoot (1 -82 9 dw/ m2) in the deepest station. Zooepiphytes biomass varies from 5 to 188 mg dw/ shoot (2-180 9 dw/ m2). A t ime delay between the sha llowest and the deepest stations has been found for all leaf features and phytoepipllyte composition and biomass, but not for zooep iphyte biomass. A time delay in terms of production and biomass maxima between shallow and deep phytobenthos communities is something general in the Mediterranean (BA~LESTEROS, 1984) . This appears to be an adaptation of different species and communities in order to couple their life-cycle to the seasonal pattern of the limiting factor (or factors) , usually light (in deep communities) and nutrient avai lability (in shallow ones); this could be also the case of Posidonia and its epiphytes. Production estimates of leaf-epiphytes vary from 70 9 C/ m2 year in shallow stations to 12 9 C/ m2 year in the deepest one, that is, between the 10 and the 20 % of the whole Posidonia meadow production .

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