In July 1988, we have observed symptoms on Xanthium italicum Moretti, similarto those provoked by Phomopsis helianthi Munt.-Cvet on sunflowers. The causative agent of these symptoms on X. italicum is Diaporthe (Phomopsis) sp. that we have arbitrarily named Phornopsis (Diaporthe) "xanthium". ' This publication describes the anamorphic and telomorphic forms of this parasite, and gives the first results concerning its biological cycle on X. italicum. The peculiarity of this fungus is to be equally pathogenic on sunflowers (results proved by ascospores tests), and to provoke on the stems of this plant identical symptoms to those developed by P. helianthi. Then, what is the relation between P. "xanthium" and P. hetianthi? The comparison of their anamorphic forms brings them together. But, on the other hand, their telomorphic forms present quite important differences. However, one of the P. helianthi biotypes isolated by Yang (1983) is characterised by typical traits of D.helianthi and of P. "xanthium". Consiclcring these various observations, and the fact that the Xanthium and Helianthus genera are hoth native to the American continent, we are presently working on the following hypothesis: could Phomosis (Diaporthe).helianthi be a form of Phomopsis (Diaporthe)."xanthitum" adapted to a new host (the sunflower)?