All three highly acidic volcanic areas examined on the Azorian island of Sao Miguel were inhabited by the red alga Galdieria sulphuraria (GALDIERI) MEROLA. The morphology and physiology of the isolates from Sao Miguel did not differ significantly from isolates collected in other parts of the world. Similar to other isolates the maximum growth temperature was about 56 °C, and growth was optimal at pH 1-2. Growth rates under auto- and heterotrophic conditions were comparable. Especially at a site in the town of Furnas extensive cryptoendolithic growth of G. sulphuraria was observed. Means of dispersal are discussed for this extremophilic alga.