Fermentation involves the synthesis of ATP in the absence of oxygen through glycolysis. It occurs under anaerobic conditions.

Three principal types of fermentation are carried out by fungi: alcoholic, lactic acid, and mixed acid fermentations.

Alcoholic fermentation involves the reduction of pyruvate into ethanol and carbon dioxide. It is best known from the classical studies on Saccharomyces (yeast). Yeast, when under anaerobic conditions, convert glucose to pyruvate via the glycolysis pathways, then go one step farther, converting pyruvate into ethanol, a C-2 compound. This process underlies the beer brewing and bread baking processes.

The biochemical process involved in alcoholic fermentation is glycolysis mediated through the Embden-Meyerhoff- Parnas pathway outlined in the figure below.

Many common molds of the genera Aspergillus, Fusarium, and Mucor are also well-known for their abilities of alcoholic fermentation, although they are obligate aerobes, requiring O₂ for other growth processes.

Many organisms will also ferment pyruvate into, other chemicals, such as lactic acid. This process is known as lactic acid fermentation.

Lactic acid fermentation are found primarily in the Chytridiomycetes, Oomycetes, and Zygomycetes.

The mixed acid fermentation is found in a small but very interesting group of obligately anaerobic Chytridiomycetes. Mixed acid fermentation has acetate, lactate, formate, ethanol, methane, CO₂ and H₂ as end products.