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The effect of environmental factors on spatial variation of the polychaete community of the alga Halopteris scoparia in Algeciras Bay (Spain) was investigated. Environmental gradients (outer and inner areas) were detected, based fundamentally on hydrodynamism, solids in suspension,% of organic matter in the sediment, and structural complexity of the plant. Polychaetes can give information on strong environmental gradients, especially pollution, but showed little discrimination among stations, due mainly to the great homogeneity generally showed by these communities. This homogeneity was the result of the quantitative importance of ubiquitous species, such as Platynereis dumerilii, Sphaerosyllis hystrix, Syllis prolifera, and Amphiglena mediterranea, among others. The outer zone, however, included a great number of species with few individuals and low dominance, among which the most characteristic was the terebellid polychaete Nicolea venustula. This species seems to have strong hydrodynamic requirements and may have been limited by excess of accumulated sediment on the alga. More of the collected species were exclusive to the outer areas, and did not penetrate into the inner bay, while most of the inner species were also found in the outer zone. As a whole, the great majority of the species were located in intermediate environmental conditions, indicating a generalist character of the polychaete species.