The pathogenicity of one isolate of Xanthomonas malvacearum (E. F. Smith) Dowson was tested under greenhouse conditions on the cotton varieties Stoneville-20, Nu-16, Gregg, Mebane, RM2 and IAC-12 and under field conditions on Stoneville-20, Nu-16, RM2 and RM4 varieties. Stoneville-20 and Nu-16 varieties showed a high degree of resistance when tested either under greenhouse or field conditions. The varieties RM4, Gregg, Mebane an IAC-12 showed to be susceptible to the isolate tested. The variety RM2 was found to be composed of resistant and susceptible plants. Only 20% of the progenies of RM2 plants selected as resistante plants when artificially inoculated under field conditions, showed to have a high degree of resistance when artificialy inoculation under greenhouse conditions.