Abstract: In the laboratory diploid pure cultures of A. ostoyae Romagn. produce fruit bodies with clampless basidia, whereas the natural fruit bodies of this species have basidia with a basal clamp. In nature the basidia develop from dikaryotic subhymenial cells with haploid nuclei and two nuclei fuse in the basidium. In pure culture the basidia develop from uninucleate diploid cells and no fusion of nuclei takes place in the basidium. Reduction of genetic material in fruit body primordia apparently precedes the development of subhymenial hyphae in nature. Such reduction does not take place in pure culture.