Bacterial blight of cotton is caused by the bacterium Xanthamonas campestris pv. malvacearum (Xcm). This infection attacks leaves and bolls and causes early leaf drop and affects boll development and normal boll opening. In Australia the CSRO breeding program has developed varieties that are resistant to Xcm infection. The use of these resistant cultivars has meant that the importance of this disease has been reduced. The resistance locus was introduced into the breeding program from the USA in the 1970's; however, the identity of the resistance gene or genes has not been fully clarified. G. barbadense or Pmia varieties are susceptible to Xcm, and a breeding program is in place aimed at introgressing Xcm resistance into the Pima cottons from G. hirsutum. Further work is needed to characterise the source of the resistance, and possibly to develop molecular markers linked to the resistance locus in order to assist with breeding efforts. In order to map this resistance locus a mapping population was set up using backcrosses between the resistant G. hirsutum variety CSSO and the susceptible G. barbadense variety Pima S-7.