The effects of freshwater sponge (Porifera) overgrowth on the fitness of adult zebra mussels (Dreissena polymorpha) were examined in East Chicago and Michigan City, Indiana harbors in southern Lake Michigan. There was significant reduction in glycogen content of sponge-covered zebra mussel soft tissue wet weight (mean=2.6%) when compared to the non sponge-covered population (mean=3.9%). Similarly, the soft tissue wet weight of sponge-covered zebra mussels (15 to 25 mm size class) was lower when compared to non-covered mussels. A comparison of the live:dead ratios of zebra mussels in both populations showed overgrowth by sponges had a significant lethal effect. These data suggest that the epibiont growth of freshwater sponges on adult zebra mussel shells negatively impacts the growth and survivability of zebra mussels.