Cystidium

: cystidia) is a relatively large cell found on the sporocarp of a basidiomycete (for example, on the surface of a mushroom gill), often between clusters of basidia.

Since cystidia have highly varied and distinct shapes that are often unique to a particular species or genus, they are a useful micromorphological characteristic in the identification of basidiomycetes.

Cystidia may occur on the edge of a lamella (or analogous hymenophoral structure) (cheilocystidia), on the face of a lamella (pleurocystidia), on the surface of the cap (dermatocystidia or pileocystidia), on the margin of the cap (circumcystidia) or on the stipe (caulocystidia).

Sometimes the cheilocystidia give the gill edge a distinct colour which is visible to the naked eye or with a hand lens.

Like gloeohyphae, they may be yellowish or clear (hyaline) and can sometimes selectively be coloured by sulphovanillin or other reagents.