An ascus (plural asci) is the spore-bearing container produced in the ascocarps of ascomycete fungi. Each ascus contains eight ascospores, produced by a meiotic cell division followed by a mitotic cell division. Asci release their spores by bursting. When one ascus bursts, it triggers the bursting of all the other asci in the ascocarp. This is an example of positive feedback.

Asci are used in laboratories for studying the process of meiosis, because the four cellss produced by meiosis line up in regular order. By modifying a gene coding for spore color, the biologist can study crossing over and other phenomena.