On a synthetic medium containing elemental sulfur (SMGS), the growth of Phomopsis viticola Sacc. decreased considerably but pycnidia formation was highly stimulated. Secondary pycnidia, developed from spores in the cirrhii themselves, were observed. Formation of pycnidia directly from spores can be described as a microcycle stage. The role of endogenous elemental sulfur in P. viticola as a differentiation inducer in the biological cycle of the fungus is discussed on the basis of the action of elemental sulfur on cellular respiration and oxidative phosphorylation.