In recent years, shellfish farming of Haliotis tuberculata, the European abalone, has started in Ireland and France. In general, cold waters protect the gastropod from bacterial and parasitic diseases but the shell colonization by boring polychaetes is an important problem causing a reduction of marketability and growth rate. During this study conducted in Brittany on animals reared with different culture techniques we showed that age and farming methods are determinant factors that influence the intensity of infestation. The polychaetes collected from abalone shells belonged to two different families: Spionidae, as referred in previous works (Clavier, 1992) and Sabellidae, reported for the first time in European abalone. Morphological studies indicated that Spionidae polychaetes belonged to different species of the genera Polydora/Boccardia. The taxonomic identification of Sabellidae at genus/species level was not reached by morphological exam, but at least two distinct species seemed to be present. Prevalence and intensity of infestation by Sabellidae were low but a single specimen was able to cause very heavy lesions of the shell. The results of this study showed that prevalence is positively linked to host age and that the farming system can strongly influence the infection intensity of boring polychaetes: very low in in-land farms, where water is filtered and hygienic procedures are routinely applied to tanks, and higher in mariculture, in particular in polyculture and areas with a low water turnover, with slow growth and performance indexes reduction