basidium

thumb|right|500px|Diagram showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.

A basidium (: basidia) is a microscopic spore-producing structure found on the hymenophore of reproductive bodies of basidiomycete fungi. The presence of basidia is one of the main characteristic features of the group. These bodies are also called tertiary mycelia, which are highly coiled versions of secondary mycelia. A basidium usually bears four sexual spores called basidiospores. Occasionally the number may be two or even eight. Each reproductive spore is produced at the tip of a narrow prong or horn called a sterigma (), and is forcefully expelled at full growth.