The distribution of polychaetes on submerged Cystoseira has been studied, mainly in the sublitoral from 0 to 2.5 meters, and at localities of different exposure. 1. 93 species belonging to 18 different families were found. Algae with no epiphytes are colonized by only a few species of polychaetes most of which can also be found in other habitats. 2. In order to develop most polychaetes require a secondary structure such as is provided by the epiphytes growing on the cystoseiras and by epizoic animals. One fifth of the species of polychaetes known from the Mediterranean occurs in the epigrowth of submerged Cystoseira. 3. The structure of Cystoseira specimen is conditioned by hydrodynamic factors of the medium. There is a denser population of polychaetes in exposed areas. The algae growing on cystoseiras can be divided into ekto-, meso- and basosessile species, and according to their degree of dependence on algal structure the polychaetes may be epi-, meso- or endobionts. 4. The conclusion to be drawn from this investigation is that most polychaetes occurring on cystoseiras need only small interstitial spaces to develop and have no specific food requirements.