Assess this feature on a squash preparation of a lamella. Look for basidia of markedly different size.

The number of sterigmata on basidia can also vary: see Basidium sterigmata number.

Choose this state if: there are two (or rarely three or four) size classes among mature basidia. The ranges of the lengths of each size class are not or only slightly overlapping. Spores produced from polymorphic basidia also have a multimodal distribution.

The presence of polymorphic basidia is often (but not always) associated with deliquescent lamellae. When basidia are polymorphic, they are also often interspersed with pseudoparaphyses (pavement cells, brachybasidia), which are sterile, non-protoplasmic elements, usually broader than basidioles.

Polymorphic basidia are found in Hygrocybe, Leucocoprinus, Coprinus, and Coprinellus and allied genera.