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 < 0.05) between infected and non-infected animals. Although the internal shell cavity was smaller in infected clams, thus affecting considerably the BCI, the proportion of dry tissue mass was not significantly different. In addition, the symbiosis can only be found in clams larger than 33 mm. M. donacium undergoes a migration from the intertidal (up to 30 mm juveniles) to the lower subtidal (post-juveniles and adults) during its life cycle, therefore the symbiosis only occurs during a later ontogenetic stage when individuals inhabit the subtidal level. This suggests that M. donacium is well adapted to the presence of the commensal. Thickness of blister walls showed differences (F= 2.834, P < 0.05) between months: average notch thickness decreases from August to December, suggesting a seasonal pattern of the infection. The economic consequences of the infection are discussed based on our results.