We compared the physiological status and biochemical composition of unionid mussels (Amblema plicata) from a riverine population with and without attached zebra mussels (Dreissena polymorpha). Unionid specimens were collected from Lake Pepin, Mississippi River, WI and MN, where both infested and noninfested unionids were experiencing local increases in water clarity. Infested specimens had higher ammonia excretion rates, lower respiration to nitrogen excretion ratios and lower clearance rates than noninfested specimens. Infested specimens also had lower carbohydrate and protein contents. Zebra mussel infested unionids had depleted energy stores and we hypothesize that they were starving, relative to noninfested individuals from the same location. This comparison helps to clarify the nature of zebra mussel impacts on unionids by showing that direct attachment is a very important component of the effect of zebra mussel populations on unionids.