The interactions between Pythium oligandrum hyphae and two types of sclerotia, i.e. the plano-convexoid sclerotium of Botrytis cinerea and the tuberoid sclerotium of Sclerotinia minor, were investigated by ultrastructural and cytochemical experiments. In the mycoparasitism of P. oligandrum, some differences in relation to sclerotium anatomy and the role of the rind layer in preventing invasion are documented. Both types of sclerotia showed neither alterations of the heavily melanised rind walls, nor direct rind wall penetration by P. oligandrum. This oomycete successfully entered B. cinerea sclerotia only through breaches at the junction of rind cells and corresponding to gaps in melanin deposits. On the other hand, none of these breaches was observed in the small sclerotia of S. minor, and P. oligandrum ingress in the sclerotia stopped at the inner rind layer. After the penetration of B. cinerea sclerotia by the mycoparasite, it extensively colonised the cortical and medulla areas by intercellular growth. The invaded tissues displayed pronounced alterations as well as some disorganisation of tissue in places. Colonisation was associated with severe chitin degradations of all host walls, which occurred even at some distance from P. oligandrum hyphae. The observation of wall thickenings in some P. oligandrum–hyphae suggests that the sclerotial cells constitute a harsh environment unsuitable for survival of the mycoparasite. These wall thickenings could be interpreted as P. oligandrum defence-like reactions.