The effects of polychaete (Lysidice ninetta, L. collaris, Nematonereis unicornis (Annelida: Polychaeta)) and limnoriid isopod (Limnoria tuberculata (Crustacea: Isopoda)) borers in the Mediterranean sea grass Posidonia oceanica (Potamogetonaceae) were assessed in the meadows of Olbia and Genoa (western Mediterranean Sea). Borer invertebrates were mainly found into the old leaf sheaths of the sea grass. At Olbia, a mean density of borers (202.6±34.8 ind m−2) significantly higher than at Genoa (23.2±7.3 ind m−2) was observed. In 3.3% of samples from Olbia, large polychaetes and isopods were also observed boring into living plant tissues. Only fractions from 0.8—3.9% of the sheath production was actually removed by borers at the two studied meadows. The fact that sheath weight loss with time was higher at Genoa than at Olbia regardless of the presence of borers suggests that other factors than the activity of such organisms are important in the sheath decay process.