The author kept 108 algal strains (Cyanophyta 53, Chlorophyta 35, Chrysophyta 20), of axenic cultures from the Kol-Algotheca in the Botanical Division of the Hungarian National Sciences Museum in the Baradla Cave, at Aggletek (Hungary) in darkness for 204-420 days under different environmental conditions. The experiments have proven that several algal strains can tolerate well the complete absence of light. Furthermore, that some algal strains show intensive development even under such conditions. These axenic cultures kept in the cave in metal boxes on inorganic medium have shown that the energy source used by these green coloured algae is not some by-product of chemotrophic bacteria, nor is it available organic material, but that it must be some kind of radiation which is able to penetrate even the metal boxes. The ability to adapt to the conditions existing in a cave is not a general characteristic of algal species, but is the capability of individual algal strains within that species. Most probably the algae living in the caves are aerophytes, terrestrial forms, and also some belonging to the edaphon. The cells were found to be smaller in the algae kept in the cave, there was almost no starch deposition in the cells, the pyrenoids were barely discernible, but the development of carotenes was more intense. Whether there are specific cave dwelling algal strains must be determined by future algological research conducted in caves. The composition of the algal floras of the caves may be equally dependent upon the chemical and physical characteristics of the biotope, as is the case in every other biotope.