Rhizopus oligosporus is a fungus of the family Mucoraceae and is a widely used starter culture for the production of tempeh at home and industrially.

As the mold grows it produces fluffy, white mycelia, binding the beans together to create an edible "cake" of partly catabolized soybeans.

It grows effectively in the warm temperatures (30–40 °C or 85–105 °F) which are typical of the Indonesian islands; it exhibits strong lipolytic and proteolytic activity, creating desirable properties in tempeh; and it produces metabolites that allow it to inhibit and thus outcompete other molds and gram-positive bacteria, including the potentially harmful Aspergillus flavus and Staphylococcus aureus.

R. microsporus produces several potentially toxic metabolites, rhizoxin and rhizonins A and B, but it appears the domestication and mutation of the R. oligosporus genome has led to the loss of genetic material responsible for toxin production.

[5] Rhizopus oligosporus strains have a large diameter (up to 43 μm) and irregular spores with widely varying volume, (typically in the range 96–223 mm3).

[5] Even after it is consumed, Rhizopus oligosporus produces an antimicrobial peptide that limits gram-positive bacteria like Staphylococcus aureus and Bacillus subtilis.

[9] For the tempeh to attain its characteristic, compact, 'cake' form after fermentation, the soybeans become compressed by the mycelia of Rhizopus oligosporus.