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Bacterial blight caused by Xanthomonas Campestris pv. malvacearum is an important disease found throughout cotton growing areas in Uganda. Its symptoms known as seedling blight, angular leaf spot, black-arm and bacterial boll rot are all found in Uganda. Aset of 8 upland cotton cultivars developed by Hunter et al. (1968), i.e. differential varieties, were used to determine the variants or races of the blight pathogen found in Uganda. Results indicated race 10 or 18 as the commonest, followed by race 7. Races 6 and 16 were also identified. About 23% of the isolates caused symptoms on all differential varieties indicating the presence of a new virulent race of the pathogen, which may be race 20. The existence of several races and appearance of new ones has implications on breeding resistant cotton lines and some of these are discussed in this paper.