Rosellinia necatrix is a pathogenic fungus which causes white root rot to potato, soybean, tea, peanut, apple and so on. It is well known that this fungus produces cytochalasin E, which has physiological activity towards mammalian cells, microorganisms and plants. We have searched in detail for new plant growth regulators in the metabolites of this fungus and succeeded in isolating of the compound designated rosellichalasin (1) together with cytochalasin E (2). This paper deals with the isolation and structural elucidation of rosellichalasin.