Ricinoleic acid has been unequivocally identified for the first time as a metabolite in laboratory cultures of Claviceps purpurea (Fr.) Tul., constituting the principal component of the estolide triglyceride oil from plectenchymatic growth forms of selected strains. The non-plectenchymatic or sphacelial form, in which most isolates of C. purpurea grow, not only contained less total triglyceride oil but the ricinoleic acid component was absent, and was replaced by linoleic and/or oleic acids. Thus the control mechanism which determines the differentiation of plectenchymatic cells into a sclerotial tissue, to which ergot alkaloid biosynthesis is confined, also initiates the biosynthesis of a unique estolide triglyceride based on ricinoleic acid. The occurrence of ricinoleic acid is thus the most precise indication of the presence of sclerotial-like tissue of C. purpurea in laboratory cultures.