Based on recent and historical collections dating back to the early 1900s, the seaweed flora in Casco Bay, Maine consists of 206 taxa: 55 Chlorophyta, 70 Phaeophyta, and 81 Rhodophyta. In 1911, Frank Shipley Collins recorded 173 taxa from Casco Bay, while 164 taxa were found during recent sampling since 1994. A comparison of the two time periods shows a 77.7% similarity; some plants have apparently disappeared (or are very rare) and others are newly recorded. Eight newly collected seaweeds represent species that are most common in estuarine and shallow embayments and have southern warm-water affinities (e.g., Hummia onusta, Striaria attenuata, Callithamnion corymbosum, Ceramium cimbricum, Chondria baileyana, Polysiphonia denudata, P. elongata, and Spyridia filamentosa); six others represent introduced taxa, including Codium fragile subsp. tomentosoides, Ulonema rhizophorum, Bonnemaisonia hamifera and its “Trailliella intricata” stage, plus Dumontia contorta, Lomentaria clavellosa, and Porphyra yezoensis f. yezoensis. The Asiatic red alga Neosiphonia harveyi, which represents another introduced taxon (i.e., 7th), was reported by Collins during the early 1900s. Comparisons of historical and present-day floras at two sites in Harpswell, Maine showed that the percent similarity values were lower and ranged from 41.5% at Basin Cove to 48.9% at Potts Point. At Basin Cove enhanced water motion and the loss of a tidal dam caused increased and altered species composition, while anthropogenic impacts due to a lobster pound diminished the flora at Potts Point. Historical floristic comparisons of seven other areas (Helgoland, Germany; the North Adriatic Sea; Denmark; Mount Desert Island, Maine; Penikese Island, Massachusetts; southern California; and the Florida Keys) are used to evaluate the stability of Casco Bay's flora and the impact of oil pollution. A comparison of Casco Bay's “composite” flora of 206 taxa with several other Northwest Atlantic sites shows that its species richness is approximately the same as that found within the Great Bay Estuarine System of New Hampshire and Maine (216 taxa), while it exceeds Penobscot Bay in mid-coastal Maine (139 taxa), Passamaquoddy Bay in New Brunswick (159 taxa), and several Northwest Atlantic islands (51–145 taxa). However, there are fewer taxa than in Newfoundland (254 taxa), the largest insular habitat within the temperate Northwest Atlantic. Casco Bay's seaweed flora is dominated by annuals (i.e., 117 vs. 86 perennial taxa), reflecting its seasonally dynamic habitats and pronounced temperature fluctuations. Annuals or opportunistic forms also dominate the oil-impacted Fore River/Portland Harbor area.