The cyanobacterium Microcystis PCC7806 fermented endogenously stored glycogen to ethanol, acetate, CO2, and H2 when incubated anaerobically in the dark. The switch from photoautotrophic to fermentative metabolism did not require de novo protein synthesis, and fermentation started immediately after cells had been transferred to dark anoxic conditions. From the molar ratios of the products and from enzyme activities in cell-free extracts, it was concluded that glucose derived from glycogen was degraded via the Embden-Meyerhof-Parnas pathway. In addition, CoA-dependent pyruvate:ferredoxin oxidoreductase, alcohol dehydrogenase, acetate kinase, and hydrogenase were present. The specific activities of these enzymes were sufficiently high to account for the rates of product formation by cell suspensions.