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Sclerotium rolfsii , the causal organism of stem rot or southern blight of groundnut, is widely distributed and has been reported to cause disease in over 500 plant species throughout the world and at least 30 in South Africa. The disease is a problem in most groundnut producing areas in South Africa and no efficient control strategy has yet been developed in this country. Previous studies on the fungus have indicated that diversity within populations of S. rolfsii can be defined by placing isolates into mycelial compatibility groups (MCGs). A total of 121 S. rolfsii isolates were collected from 15 localities and seven plant species throughout South Africa and paired against each other. Thirteen MCGs were identified, some containing isolates from the same host plant or geographic area, suggesting a possible relationship between MCG and host plant or locality. Other MCGs, however, contained isolates from a variety of hosts from various localities. The population of the fungus in South Africa is diverse, with vague associations between MCG, host and geographical distribution.