The effect of sponge encrustation on swimming ability of Chlamys hastata was determined by investigating swimming behaviour, scallop morphometry, and energy expended during swimming with and without commensal epibionts. Scallops swam significantly longer after sponge encrustation was removed from their shells, but no significant differences were detected in swimming elevation or distance. Scallops with sponge encrustation showed no adductor muscle hypertrophy or changes in shell morphometry compared to scallops without encrustation. However, C. hastata did exhibit scaling relationships associated with maximizing swimming efficiency. Specifically, shell width and adductor muscle mass were positively allometric with shell height, while shell mass was negatively allometric with shell height. Scallops increased their energy expenditure (both aerobic and anaerobic) during valve-clapping, but no significant difference was detected between unencrusted (43·0 μmol adenosine triphosphate [ATP] consumed during a two min escape swim) and sponge-encrusted (40·0 μmol ATP) scallops. Scallops in both treatments derived 86% of the energy used for swimming from anaerobic sources. The lack of substantial differences between scallops with and without commensal sponges is partially explained by the observation that even heavy sponge encrustation increases the immersed weight of the scallop by only 5%. The presence of a sponge encrustation does not appear to inhibit swimming by this scallop species.