The fungus Thielaviopsis basicola (Berk. & Br.) Ferraris exhibited considerable variability when grown on potato dextrose and V8 juice agars. Variants were frequently obtained from single chlamydospore cells. In general, many different cultural variants were obtained from the grey wild biotype. These arose from single chlamydospore cell isolations, from sector and patch variants, and from colonies which initially were of single spore origin. The variants differed from their respective parents and from each other in macroscopic and microscopic characteristics including colony appearance, color, zonation, growth rate, production of spores, and shape and number of cells in the chlamydospores. Some variants were less pathogenic than the parents from which they arose; in others pathogenicity was not significantly changed, and others exhibited higher pathogenicity.Contrary to earlier reports, it was found that the typical brown type can be derived in culture from the grey wild biotype. This brown type is similar to the brown wild biotype in cultural behavior and in relative pathogenicity to red clover.