The influence of inoculum potential of Fusarium oxysporum f. vasinfectum on cotton wilt was investigated. The tempo of disease development in a susceptible variety of cotton (Karunganni 5) increased with a rise in the inoculum level from 0.5% to 10% of an oatmeal-sand culture in unsterilized soil, but further increase caused a marked delay. This effect was not observed in sterilized soil, in which wilt developed rapidly at all inoculum levels. In a resistant variety (Madras Cambodia Uganda 1) wilt developed only in 10% inoculum; at lower levels a few plants showed initial symptoms but recovered. The addition of sterilized oatmeal or of sucrose at the 10% inoculum level delayed wilt development. Incubation of a mixture of 2 % inoculum and soil for 45 days hastened disease development, but subsequent amendment of the mixture with oatmeal again slowed down the tempo of wilt development, although the soil population of the Fusarium was increased by the addition of fresh oatmeal. Organic nutrients in soil are therefore considered to influence the pathogenicity of this fungus, and soil micro-organisms are thought to be concerned in this effect.