The inland algal flora of Acadia National Park was studied based on over 200 samples from 119 localities. A total of 160 algal genera were found, representing 12 algal phyla. Among these were 140 new generic records for the Park. The most frequently encountered taxa were desmids and diatoms, and the genera and species in these groups were acidophilous, reflecting the low pH of the ponds, lakes, and streams of the Park. Ponds and streams were the most heavily sampled habitats in the Park, and coincidentally had the highest diversity (103 and 91 genera, respectively). Based on other similar studies, we estimate that the Park likely contains as many as 1000 algal species within its boundaries, indicating that the diversity is exceptionally high for an area of this size.