Evidence is presented that the biogenesis of sclerotia in Sclerotium rolfsii is associated with lipid peroxidation. Sclerotial initials show 100-fold increase in lipid peroxides of their total lipids as compared with young mycelia grown under reducing conditions (in 2-mercaptoethanol) in dark and without Fe(II). There was a direct relationship between the number of sclerotia formed and lipid peroxidation levels in the mycelial colonies. Lipid peroxides of possible membranous and cytoplasmic origin were found in sclerotial exudate. In this paper a new approach is advanced for the understanding of the mechanism of sclerotial formation in Sclerotium rolfsii and in other fungi. The data in this work on lipid peroxidation – the most frequently evoked consequence of oxygen free radicals – as well as the data of past experiments, strongly suggest that this phenomenon may be associated with oxidative stress caused by growth conditions.