Ascomycota is the largest phylum of fungi by number of described species.
Ascomycota, commonly called the sac fungi, contains more than 64,000 described species and forms one half of the fungal subkingdom Dikarya alongside Basidiomycota. Its members include familiar organisms such as baker’s yeast, morels, truffles, cup fungi, many molds, and most lichen-forming fungal partners.
The group is defined by the ascus, a microscopic sexual structure in which ascospores develop. During the sexual cycle, genetic material undergoes meiosis, usually followed by a mitotic division, commonly producing eight spores within each ascus. These spores may be enclosed in visible fruiting bodies called ascocarps, which can be cup-shaped, flask-shaped, closed, or highly irregular.
Species counts change as scientists describe new fungi and revise classifications using DNA evidence. The figure therefore represents described diversity rather than the total number of species that exist. Basidiomycota is another major fungal phylum, but it is distinguished by basidia, the spore-bearing cells found in mushrooms such as shiitakes and puffballs.