THE interaction of two haploid cells of opposite sex to produce the diplophase is a phenomenon well known in heterothallic species of the Hymenomycetes and of the smut and rust fungi. Buller1 has recently shown that a diploid mycelium of Coprinus lagopus, on coming into contact with a haploid mycelium of that species, transforms the latter mycelium into the diploid condition. He has introduced the word diploidisation “to designate in the Hymenomycetes the process by which a haploid cell, or mycelium, is converted into a diploid cell, or mycelium, by the formation of conjugate nuclei within the cell or mycelium”. An extension of the application of this term to designate a similar process in other groups of the Basidiomycetes, such as the smuts and rusts, does not appear inappropriate.