In previous work, we observed wide levels of diversity among strains of Sclerotinia sclerotiorum in the susceptibility of their sclerotia to colonization by the mycoparasite Coniothyrium minitans. Here, we investigated a possible relationship between the level of in vitro susceptibility of the strains and the morphological traits of their sclerotia, with the aim of providing a simple predictive tool for field assessment. We focused on the average thickness of whole sclerotia and on that of the melanised cortical tissue that the mycoparasite needs to penetrate to colonize the medullar tissue. Significant differences were found among strains, with ranges of 0.80-1.72 mm and 51-84 μm, respectively, for sclerotium size (P < 0.001) and for cortex thickness (P = 0.05). However, sclerotium size was not correlated to any of the susceptibility indices examined for the strains (P values > 0.26; R² < 0.06). Cortex thickness was significantly correlated (P = 0.019; R² = 0.32) to the average frequency of detection of C. minitans in inoculated sclerotia but not to other susceptibility indices. These results suggest that other factors (possibly related to the biochemical composition of the tissues) play a determinant role in susceptibility of sclerotia to C. minitans. As a high level of intra-strain variability was observed (both in terms of morphological traits and susceptibility of sclerotia), work is under way to characterize individually larger numbers of sclerotia for selected strains.