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The sunflower (Helianthus annuus L.) is one of the most important crops grown for edible oil. Sclerotinia sclerotiorum (Lib.) de Bary is a common and widespread pathogen of sunflower. In the present study the reaction of 35 genotypes, including recombinant inbred lines and their parents, M7 mutant lines developed by gamma irradiation, and some genotypes from different geographical origins, were evaluated against eight S. sclerotiorum isolates in controlled conditions. The proportion of the subsequent basal stem lesions was measured 3 days after inoculation. Highly significant differences were observed among sunflower genotypes and S. sclerotiorum isolates, with the isolates interacting differentially with sunflower genotypes. Two genotypes had high partial resistance to all S. sclerotiorum isolates, whereas others were susceptible to all isolates. Isolates of S. sclerotiorum differed in virulence to host genotypes. Some genotypes showed specific interactions with S. sclerotiorum isolates, being resistant to some isolates but susceptible to others. Recom­binant inbred lines used in this study showed different reactions to eight isolates of S. sclerotiorum when compared with their parental lines. The isolate-specific and isolate-nonspecific partial resistant genotypes identified in present experiments should be used in crossing programmes for breeding of durable resistance to Sclerotinia basal stem disease.