Rhizoctonia solani Kühn [teleomorph: Thanatephorus cucumeris (Frank) Donk.] is an important fungal pathogen widespread in all potato growing areas of the world that causes stem canker and black scurf of potato (Solanum tuberosum L.). The aim of this study was to find a simple and reliable technique for determining the pathogenicity of Rhizoctonia solani isolates. Sixty (60) isolates of R. solani obtained from sclerotia on potato tubers, collected from different market of Agadir and Casablanca regions (Morocco), were studied for their morphology, pathogenicity and molecular characteristics. They were morphologically characterised by the production of sclerotia and moniloïd cells, and by the mycelium growth capacity at 15, 20, 25, 30 and 35°C. This morphological characterisation leads to three groups of isolates. The first group Original Research Article