We found 19 species of freshwater mollusks (seven bivalves, 12 gastropods) in the Plummers Island area, Maryland, bringing the total known for the Middle Potomac River to 42 species. We found seven species not previously known from the Middle Potomac River, but we failed to find 23 species that had been reported there previously. The new records probably reflect our searching in backwater and woodland pools, habitats that had been poorly searched previously. Species not found but represented by historical records (including 15 bivalves, of which 13 are Unionidae) may represent extirpations or species missed owing to low search effort for mussels. The introduced bivalve Corbicula fluminea, apparently absent from the Middle Potomac in 1978, was the third most numerous species collected in our survey. We found three bivalve species not previously reported in the Middle Potomac: Utterbackia imbecillis (Unionidae), and Pisidium compressum and Pisidium nitidum (both Sphaeriidae).