Verticillium wilt is currently the most important disease causing losses to the cotton crop in South Africa. Only defoliating type strains could be isolated. These isolates however, differed slightly in virulence to cotton. Strains collected from the Lower Orange River area were more virulent than any other isolates collected from the other areas. These strains also had different isozyme profile in the a-B-esterase systems. Indications are that these more virulent strains emerged due to selection pressure on the pathogen population, caused by monoculture of one cotton cultivar and several other agronomical practices. Progress are being made with a breeding programme for resistance to Verticillium wilt in South African cotton cultivars.