The variability of X. malvacearum was studied under greenhouse conditions with isolates obtained from the principal cotton growing regions of the State of São Paulo. The isolates were introduced into the tissues by scratching the under side of the leaves with a bamboo pick previously dipped in a bacterial suspension. For the evaluation of the symptoms a scale based on five infection types was used. As a result of these tests, the isolates were grouped into three races of X. malvacearum respectively races 3,8 and 10. The IAC RM3 71/523 line showed resistence to race 3, while (Acala x Nu-16) 71/213 and IAC 12-2 71/170 lines showed resistence to races 3,8 and 10. The susceptible cotton varieties IAC RM3, IAC 13-1 and IAC 12-2, cultivated in São Paulo State, reacted equally to the three races mentioned.