Thirty-three Polychaeta species, belonging to twelve families, have been ascertained in an association forming a thick and wide crust on the masonry wall of a forced marine water pipe, serving some plants at Augusta (Siracusa). Three species are new to the ltalian coasts, and Procerastea perrieri is new to the Mediterranean. Hydroides norvegica is the predominant form. The development of the association has been remarkably influenced by the serious pollution of the marine water by oily wastes.