A basidium (pl. basidia) is a microscopic sporangium (a spore-producing structure) found on the hymenophore of fruiting bodies of basidiomycete fungi which are also called tertiary mycelium, developed from secondary mycelium. Tertiary mycelium is highly-coiled secondary mycelium – a dikaryon. The presence of basidia is one of the main characteristic features of the Basidiomycota.