The aim of this study is to analyse the co-occurrence of the encrusting sponge Iophon proximum and other sessile organisms on the shells of the Patagonian scallop, Psychrochlamys patagonica, and the relationship between scallop size and the presence of this sponge. Scallops were collected from four beds distributed along the 100-m depth isobath on the continental shelf off Argentina (south-western Atlantic). Statistically significant avoidance was observed between I. proximum and all other groups of sessile invertebrates. Co-occurrence of bryozoans, foraminiferans, folliculinid ciliates, hydrozoans, octocorals and spirorbid, serpulid and sabellarid polychaetes on valves not monopolized by I. proximum produced significant clumping, which, however, almost ceased to occur when only valves without this sponge were analysed. In Patagonian beds, valves monopolized by I. proximum were significantly smaller than those in which the sponge was absent (valve height, mean ±SE, with I. proximum: 54.1 ±1.74 mm, without I. proximum: 60.4 ±0.62 mm). This is the first study to report a significantly smaller size in sponge-covered scallops compared to those without sponges.