Wilt of cotton caused by Fusarium oxysporum f. vasinfectum was found at Presidente Prudente, State of São Paulo. More than 60 varieties from collections maintained by the Cotton Department of the instituto Agronômico were tested for suscetibility when grown artificially in large test tubes in Hoagland's solution made solid by addition of agar. Ten varieties promised to be good material in a first screening. From sulphuric acid delinted cotton seeds followed by washing and desinfestation by calcium hypochoride, Fusarium oxysporum f. vasinfectum. (32) was isolated from what it is bellieved to be living internal tissues of seeds obtained from wilted plants in the field. Six out of a thousand seeds tested carried consistently the fungus. In the States from the work, by Elliott (19, 20) and later by Taubenhaus and Ezekiel (33) higher percentages were secured when they investigated the problem.