We compared the physiological rates and biochemical composition of dreissenid mussels (Dreissena polymorpha) from a riverine population that were attached to unionid mussels (Amblema plicata) or to rocks. Specimens were collected from Lake Pepin, Mississippi River, Wisconsin and Minnesota. Unionid-attached specimens had greater oxygen uptake and ammonia excretion rates than did rock-attached specimens. However, unionid-attached specimens also had lower carbohydrate and lipid contents. Our results indicate that there are physiological costs of attaching to unionids, at least under some circumstances. Interspecific competition for a limited food resource may negatively impact not only the unionids, but the attached dreissenids as well. This conclusion challenges the assumption that dreissenids benefit from attaching to unionid mussels.