Spontaneously-occurring hypovirulence in the tan sclerotial isolate S10 of Sclerotinia sclerotiorum from sunflower in Canada was characterized and compared to another hypovirulent isolate Ep-1PN of S. sclerotiorum from eggplant in China. Hypovirulent isolates derived from S10 were purified by single hyphal tip isolations from colonies of S10 showing abnormal growth on potato dextrose agar (PDA) and tested for pathogenicity on leaves of canola (Brassica napus cv. ‘Westar’). These abnormal isolates differed from the virulent isolate wtS10 derived from a normal colony of S10 by the reduced hyphal growth, induced production of abnormal hyphal branches, enhanced production of brown pigments, reduced sclerotial formation on PDA, reduced oxalic acid accumulation in potato dextrose broth, and reduced pathogenicity on canola. Vegetative transfers revealed that the hypovirulence phenotype of the hypovirulent isolate S10-2A-11 was stable. This preliminary in vitro transmission test indicated that the hypovirulence in the isolate S10-2A-11 was transmissible but at a lower rate than that of the hypovirulent isolate Ep-1PN. Double-stranded ribonucleic acids (dsRNAs) were detected in isolate Ep-1PN, but not in hypovirulent and virulent isolates derived from S10. The existence of dsRNA-free hypovirulence in S10 progenies suggests that another hypovirulence mechanism may exist in S. sclerotiorum.