The surf clam Mesodesma donacium is one of the most valuable commercial species in Chile and Peru. Populations of the northern and central area of the geographic range can be decimated during El Niño events (Arntz et al. 1987). Although boring polychaetes represent a threat to native and commercial molluscs (Moreno et al. 2006), the effects of the association between Polydora bioccipitalis and M. donacium has not been studied yet. Clams covering the whole size range were sampled monthly at Hornitos (northern Chile) and classified into four infestation levels (0 to III) to study: i) the relationship between the prevalence of infestation and host size, ii) timing of infestation events and iii) effects on body condition (BCI) and reproduction (GSI). Additionally, different staining chemicals (alizarin red, calcein and strontium chloride) were tested for suitability of growth analyses, resulting in calcein to be the best stain. This method enabled comparisons of short-term growth differences between reared clams at different infestation levels. Furthermore, effects of the infestation on digging ability were tested as well in reared clams. The relationship between prevalence and host-size was well described (r2 = 0.959) by a logistic regression model: clams under 34 mm in shell length where never infested, which could be related to habitat segregation between juvenile and adult clams. A rapid increase of prevalence in medium-size clams was explained by gaping in shell closure. Main infestation events were coupled with BCI and GSI drops. Hence, infestation takes place during energy-demanding periods of the host after spawining events. As previously observed by Lleonart et al. (2003), effects of infestation depend on the severity of infestation and the host size: significant negative effects on body condition, growth and digging time were found only in heavily infested adult clams. No differences in GSI where observed between clams with distinct infestation levels, which suggests that even if infestations affect body conditions, the gametogenic cycle apparently is not affected and follows the natural rhythm. As the percentage of heavily infested animals of this population was low (7.6% of all infested clams) it seems to be capable of sustaining the thresholds of stress induced by this symbiosis. This, however, could significantly change under environmental stress and play a role in mass mortality events commonly observed in this species, given previous observations on the Polydora spp. infestation ability (Tinoco-Orta & Cáceres-Martínez 2003) and the traits of M. donacium populations (Arntz et al. 1987).