The surf clam Mesodesma donacium is one of the most valuable shellfish species in Chile. It exhibits a high prevalence of infestation (31.4 %) by the shell-inhabiting polychaete Polydora bioccipitalis. 80 specimens covering the whole size range were collected monthly from Hornitos (Northern Chile) to determine (i) the shell size of first infestation and (ii) the effect of infestation on body condition index (BCI = dry tissue mass/internal cavity volume*100). The duration of the symbiosis was determined by measuring the thickness of the blister walls formed inside the bivalveŽs shell. Preliminary results indicate that the growth rate and BCI of the host was significantly different (t-test, P