ABSTRACT. – A number of intensive surveys were performed from 2002 to 2008 to implement the distribution data of the genus Ophryotrocha in Italian harbour and lagoon environments. The occurrences of O. hartmanni, O. l. labronica, O. macrovifera, O. p. puerilis, O. robusta and O. japonica were confirmed, and the first records of O. adherens in Italy and O. diadema in Europe were documented. The three most common Ophryotrocha were O. l. labronica, O. japonica and O. p. puerilis. The other species were rarer. Data were integrated with literature records, and some hypotheses which were proposed to justify the wide, patchy distribution of most Mediterranean Ophryotrocha were discussed. It was previously suggested that isolated populations could be remnants of a wider continuous distribution, the result of anthropogenic dispersion by means of sea traffic, and/or the by-product of a heterogeneous sampling effort in different biogeographic areas. It appeared that the last two hypotheses and the dispersion through waves, currents and/or storms could explain the distribution patterns of certain species. The high incidence of the most common species in the investigated area and the scant information outside the Mediterranean suggested that the distribution of Ophryotrocha species could be wider and more continuous than that recognized today.