Abstract The dark anaerobic metabolism of Nostoc sp. strain Cc, isolated from coralloid roots of Cycas circinalis, was studied. As soon as cells were transferred to dark anaerobic conditions, exogenous glucose was actively metabolized to yield acetate as the sole fermentation product. The absence of other fermentation products and the stoichiometric ratio of about 3 mol of acetate per mol of glucose consumed strongly suggested that strain Cc is able to perform homoacetic fermentation. The same type of fermentation was also observed at the expense of endogenous carbohydrates as well as under reduced partial pressure of O2. The homoacetic fermentation allowed the strain to survive but not to grow.