Invasion of North American waters by Dreissena polymorpha and D. rostriformis bugensis has resulted in declines in native North American Unionoida mussels. Dreissenid mussels biofoul unionid mussels in large numbers and interfere with unionid movement, acquisition of food and ability to open and close their shells. Initial expectations for the Great Lakes were that unionids would be extirpated where they co-occur with dreissenids, but recently adult and juvenile unionids have been found alive in several apparent refugia. These unionid populations may persist due to reduced dreissenid biofouling in these areas, and/or due to processes that remove biofoulers. For example, locations inaccessible to veligers may reduce biofouling and habitats with soft substrates may allow unionids to burrow and thus remove dreissenids. Here, biofouling was measured by deploying caged unionid mussels (Lampsilis siliquoidea) at 36 sites across the western basin of Lake Erie to assess spatial variation in biofouling and to identify other areas that might promote the persistence or recovery of native unionid mussels. Biofouling ranged from 0.03 – 26.33 g per mussel, reached a maximum in the immediate vicinity of the Maumee rivermouth, and appeared to primarily consist of dreissenid mussels. A known mussel refugium in the vicinity of a power plant near the Maumee rivermouth actually exhibited very high biofouling rates, suggesting low dreissenid colonization is unlikely to be the primary cause of unionid survival in this refugium. The southern nearshore area of Lake Erie, near another refugium, also had very low biofouling. A large stretch of the western basin appeared to have low biofouling rates and muddy substrates, raising the possibility that these open water areas could support remnant and returning populations of unionid mussels.