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This paper reports the discovery of endolithic algae from vertical limestone cliffs associated with the Niagara Escarpment, southern Ontario, Canada. The endolithic habitat forms a distinctive dark green layer 1-3 mm below the surface of the porous dolomitic limestone. Samples of the algal layer were isolated from freshly cleaved rock samples using sterile probes and grown on a solid (agarized) standard algal mineral medium supplemented with soil extract. A diverse assemblage of organisms grew on the agar plates, including bacteria, fungi, blue-green algae (cyanobacteria), green algae, yellow-green algae, and occasionally the protonemata of mosses. Isolated algae belong to unicellular, colonial, and filamentous morphological types. Algal genera that have been identified include seven blue-green algae (Cyanophyta or Cyanobacteria), six green algae (Chlorophyta), and one yellow-green alga (Xanthophyceae). Six of the algal genera found in Ontario rocks also occur inside rocks of the Colorado plateau in northern Arizona, and one genus also occurs inside Antarctic rocks.