As one of the two common genera of molds on fruits and other foods such as grain, wheat, and bread, species of the genus Aspergillus are distinguished from Penicillium species by the origin of their spore-bearing stalks or conidiophores. In Aspergillus, the conidiophore arises from a foot-cell, a vegetative mycelium.

The foot-cell is broader than the filament of the mycelium, and when the spores are ripe, it usually does not contain protoplasm. At the end of the non-septate (no cross walls) stalk, a swollen vesicle bears a large number of little stalks or sterigmata. Secondary branches or sterigmata bear the numerous spores.

Aspergillus species are common, widespread, and occur naturally in soils and on plant matter. Most of the species, including the most ubiquitous A. niger, prefer relatively warm habitats ranging from 35 to 40 degrees Celsius for an optimal temperature. In microbiological and tissue culture laboratories, this species is one of the most troublesome as a contaminating mold. Several human diseases are tied to inhaling Aspergillus spores, especially if the person is suffering from an impaired immune system and the possibility of forming mycotoxins.

Not all Aspergillus molds are considered a nuisance to humans. In fact, the koji mold (A. oryzae) produces kojic acid, a metabolite that possesses antibacterial and antifungal properties and is very important in Japan for the production of sake (rice wine), miso (soybean paste), shoyu (soy sauce), amasake (sweet rice milk), and shouchu (distilled rice, brown sugar, or sweet potato liquor, which is similar to brandy).