In the Transvaal, two epidemiologic patterns of Sporothrix infection occur in man. Evidence gathered from nature, the clinic and the laboratory suggests that these patterns are not the result of either a fixed strain specificity or a random mutation. The differences represent a developmental trend, determined by environmental factors, which gradually transform the wild strains of Sporothrix schenckii into variants resembling the earlier descriptions of Sporothrix beurmannii. This change, moreover, is regular and predictable.