Algological investigations carried out in the Mammoth Cave, Kentucky, revealed the presence of twenty-seven taxa representing all divisions of the Algae except the Pyrrhophyta and Phaeophyta; diatoms although observed in the samples were not dealt with in the present paper. One species, Oscillitoria clausiana spec. nov. and a form Lyngbya pusilla fa. tenuior fa. nov., both belonging to the Cyanophyta are new to science. In addition, several other rare and interesting algae were found. A comparison is made between the algal flora of the Mammoth Cave and algae found in several European caves. The ecology of the cavernicole algae is discussed.