Systematic analysis of the distribution of Polychaetes on soft bottoms was carried out at depths ranging from 1,5 to 20 m along a tract of northern Lazio coast. 52 species were found and identified. A gradual change in the Polychaete communities was observed according to the sediment type and to the depth. The different grain-size and nature of the sediment influence the presence or the absence of some species in nearby areas. At a depth of 20 m some species become less frequent or disappear while other species which prefer soft bottoms at greater depth gradually appear there.