Four fungal species were recorded in vitro as potential mycoparasites of sclerotia of Sclerotium cepivorum, the causal agent of onion white rot. Parasitised sclerotia appeared shrunken and decayed and failed to germinate. Light and scanning electron microscopy revealed colonisation of the internal structure of the sclerotium by the mycoparasites. The medullary region was progressively destroyed but the rind cells remained relatively intact. The surface of parasitised sclerotia was extensively covered by spores or fruiting bodies of the mycoparasites. The results indicate that the four test fungi show potential for biological control of the disease.