Ascomycota, a large phylum of fungi (kingdom Fungi) containing over 64,000 species. Ascomycetes are characterized by a saclike structure, the ascus, which contains four to eight ascospores in the sexual stage. The sac fungi are separated into subgroups based on whether asci arise singly or are borne in one of several types of fruiting structures, or ascocarps, and on the method of discharge of the ascospores. Many ascomycetes are plant pathogens, some are animal pathogens (Aspergillus), a few are edible mushrooms, and many live on dead organic matter as saprotrophs. See also list of fungi.

The largest and most commonly known ascomycetes include the morel (see cup fungus) and the truffle. Other ascomycetes include important plant pathogens, such as those that cause powdery mildew of grape (Uncinula necator), Dutch elm disease (Ophiostoma ulmi), chestnut blight (Cryphonectria parasitica), and apple scab (Venturia inequalis).

Perhaps the most indispensable fungus of all is an ascomycete, the common yeast (Saccharomyces cerevisiae), whose varieties leaven the dough in bread making and ferment grain to produce beer or mash for distillation of alcoholic liquors; the strains of S. cerevisiae var. ellipsoideus ferment grape juice to wine.

Another economically significant genus is Penicillium. In 1928 Scottish bacteriologist Alexander Fleming first observed that colonies of the bacterium Staphylococcus aureus failed to grow in those areas of a culture that had been accidentally contaminated by the green mold Penicillium chrysogenum (then known as P. notatum). He isolated the mold, grew it in a fluid medium, and found that it produced a substance capable of killing many of the common bacteria that infect humans. Australian pathologist Howard Florey and British biochemist Ernst Boris Chain isolated and purified penicillin in the late 1930s, and by 1941 an injectable form of the drug was available for therapeutic use.

Neurospora, a genus of widespread species, produces bakery mold, or red bread mold. It has been used extensively in genetic and biochemical investigations. Xylaria contains about 100 species of cosmopolitan fungi. X. polymorpha produces a club-shaped or fingerlike fruiting body (stroma) resembling burned wood and common on decaying wood or injured trees.

Cordyceps, a genus of more than 300 species within the order Hypocreales, are commonly known as vegetable caterpillars, or caterpillar fungi. C. militaris parasitizes insects. It forms a small, 3- or 4-centimeter (about 1.3-inch) mushroomlike fruiting structure with a bright orange head, or cap. A related genus, Ophiocordyceps, includes the zombie-ant fungus (O. unilateralis) and commonly infects ants and other arthopods.

The genus Claviceps includes C. purpurea, the cause of ergot of rye and ergotism in humans and domestic animals.

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Earth tongue is the common name for the more than 80 Geoglossum species of the order Helotiales. They produce black to brown, club-shaped fruiting structures on soil or on decaying wood.