Thirty isolates of Sclerotinia minor were collected from a 4 ha commercial lettuce crop at Holland Marsh, Ontario, and examined for the presence of transmissible hypovirulence. Three slow-growing isolates with atypical colony morphology were less virulent (P = 0.05) on detached lettuce leaves than other isolates and were considered hypovirulent. These thirty isolates comprised three mycelial compatibility groups with the five least virulent isolates producing variable interactions. Hypovirulence was transmissible among some isolates in culture and on lettuce tissue. When a mycelial suspension of a hypovirulent isolate was sprayed on lesions on lettuce leaves caused by a compatible virulent isolate, there was a reduction in lesion size and number of sclerotia produced in comparison to control treatments. Virulent isolates converted to hypovirulence retained the hypovirulent phenotype after repeated subculturing. All isolates were tested for the presence of double-stranded ribonucleic acid (dsRNA), which is associated with transmissible hypovirulence in Sclerotinia sclerotiorum. Eleven isolates, including some virulent and all hypovirulent isolates, tested positive for dsRNA. However, conclusions could not be drawn concerning associations between certain dsRNA segments and hypovirulence in S. minor because of the variability in the recovery of dsRNA. Results of this study suggest that transmissible hypovirulence has potential for use as a management strategy for diseases caused by S. minor.