The benthic marine algal flora of Cobscook Bay, ME_, which is located near the mouth of the Bay of Fundy and is contiguous with Passamaquoddy Bay, NB, Canada, consists of 148 taxa, including 38 green, 46 brown, and 64 red algae. The seaweed flora of Cobscook Bay contains 47% of the total taxa (315) known from the Bay of Fundy and 37% of the 403 taxa recorded from the northeast coast of North America. Cobscook Bay's floristic affinities range from 63.8%) (Bay of Fundy) to 74.3%» (Casco Bay), with Passamaquoddy Bay having a 68.8%) affinity. Of the 37 sites in Cobscook Bay, Eastport Harbor (70 taxa) and Reversing Falls Park (51 taxa) had the highest number of species, and the lowest was found at one Lubec site near Lead Mine Road (only “brackish” water taxa found). The Bay's overall (R C)/ P floristic ratio ([red green]/ brown algae) is 2.2, which indicates a cold-temperate flora. Six introduced seaweeds were found in the Bay. The expanded vertical distribution of intertidal (and subtidal) taxa within Cobscook Bay may be associated with extreme tides and a high incidence of summer fog. Kelps in some Cobscook Bay areas were larger than open coastal plants, reflecting the effect of protected embayments, strong currents, and high levels of nutrients, as demonstrated by Saccharina longicruris at Wilbur Neck, with an average length of 4.6 m. Saccharina latissima, growing in areas with strong tidal currents at Reversing Falls Park, had elongate and narrow blades similar to the narrow-bladed S. latissima f. angustissima known only from mid-coastal Maine. Dwarf limicolous fucoids (i.e., Fucus species “muscoideslike”) were found in some sandy salt marsh habitats similar to those described at a few scattered Canadian Maritime and Gulf of Maine sites.