Sipunculids have been used as ecological bioindicators in the areas of the harbour of Piombino (Livorno, Italy), the near pier in Follonica (Grosseto) and the harbour of Portoferraio (Elba Island, Livorno). The species of the collected samples were identified by molecular analysis of the 18S rRNA sequence: a genetically homogeneous population of Phascolosoma granulatum Leuckart, 1828, was found to be present in all the three analysed areas. The sipunculids, collected from different sites of the three areas exposed to identical environmental conditions (depth, temperature and solar radiation), were analysed in parallel to quantify the expression of HSP70, a molecular biomarker of ecological perturbation and contamination. This analysis showed that the largest amount of HSP70 is expressed in specimens taken from sites of the Piombino harbour, an area particularly exposed to industrial waste. The expression of an isoform larger in size than the conventional HSP70 was highlighted in these specimens.