Flammulina velutipes, unlike many basidiomycetes, produces homokaryotic oidia on both homokaryotic and dikaryotic mycelia. It has bifactorial incompatibility, with clamp-connexions on the dikaryotic hyphae. The oidia germinate well and cause homing of both monokaryotic and dikaryotic hyphal tips. The production of oidia by dikaryons has been investigated by using morphological markers. The oidia are formed in chains on clumps of aerial hyphae which develop a short distance behind a clamp-connexion. Some dikaryons form oidia of only one genotype. The genotype of the nucleus dividing in the main hyphal axis rather than in the clamp-connexion itself may account for this.