a white crystalline organic solid of the molecular formula C₂₈H₄₄O belonging to the steroid family. It is found only in fungi (e.g, Saccharomyces and other yeasts and Claviceps purpurea, the cause of ergot, a fungal disease of cereal grasses) and is chemically related to cholesterol. Ergosterol is converted by ultraviolet irradiation into ergocalciferol, or vitamin D₂, a nutritional factor that promotes proper bone development in humans and other mammals.

Its relationship to vitamin D was established in 1927, when it was shown that an irradiated sample of ergosterol could be used to alleviate rickets, a deficiency disease of bone caused by lack of vitamin D in the diet. Commercially, ergosterol is produced from yeast and then converted into vitamin D₂, which is used as a food supplement. See also vitamin D.