THE life -history prrfyust fungus is usually regarded as sufficiently stable to have some diagnostic value. Recent knowledge shows, however, that there are exception. Malcolm Wilson (Proc. Roy. Soc. Edin., B, 63,part 2, No. 11, 177 ; 1948) reviews the literature dealing with instability, and describes four British cases where uredosori contained a proportion of teleutospores. All the unstable species now known belong to the Pucciniaceæ, a relatively advanced family of the rust fungi. The taxonomic position of their host species also agrees with their relatively advanced condition. The interesting theory that unstable species lead to the production of micro -Puccinia forms is now formulated, and seems to provide a useful insight into the phylogeny of these organisms. Special environmental conditions appear to induce instability of life -history, and this itself is interesting feature.