Abstract: The freshwater blue-green alga Phormidium foveolarum Gem. was grown on medium containing hydrocarbons. Measurements of O2 consumption during degradation of the N-tetradecane by Phormidium foveolarum Gom. and residues of the hydrocarbon substrate were made by Warburg manometry and gas-chromatography. The results show that: (1) Phormidium foveolarum Gom. has ability to absorb and degrade the N-tetradecane and then the concentration of N-tetradecane decreased. (2) Under the dark condition the O2 consumption increased with the reaction time for hydrocarbon degradation. (3) Notable changes in cell contents of Phormidium foveolarum Gom. have been observed after it has continuously grown on the substrate containing hydrocarbon. The oil droplets appeared and more large granules have been found in the cells.