Breeding cotton for resistance to all USA races of Xanthomonas campestris pv malvacearum (Smith) Dye has been successful when resistant genotypes were selected after inoculation with a mixture of four races. The discovery of new highly virulent isolates of the pathogen in Africa led to an investigation on the efficacy of using mixed inoculum of these HV isolates in screening for bacterial blight resistance. Three cultivars, 'Tamcot CAMD-E', 'Stoneville 825', and 'S295' were inoculated at the seedling stage with single isolates and with different mixtures of isolates HV1, HV3, HV7, Sudan, and US race 18. The underside of cotyledons were inoculated by scratching with a toothpick after dipping the toothpick in inoculum. Fifteen days after inoculation, disease reactions were graded on a scale of 1 (immune) to 10 (highly susceptible). 'S295' is the only variety with high resistance to the HVl isolate. 'Stoneville 825', is susceptible and 'Tamcot CAMD-E' is resistant to US race 18. HV1, inoculated as a single isolate, was the most virulent isolate on the susceptible cultivars 'Stoneville 825' and 'Tamcot CAMD-E'. When HV1 was included in a mixed inoculum, disease grades on the susceptible genotypes were reduced by 50 percent compared to grades obtained with HV1 alone. Disease grades appeared to be reduced most when one of the isolates in a mixture with HV1 was avirulent. The reduction in disease expression caused by HV1-mixed inoculum might be due to competition, antagonism, or a more rapid triggering of host defense reaction by the less virulent strains in the mixture. Screening for resistance to the HV1 isolate requires a single inoculum as opposed to the mixed inoculum used in screening for resistance to US races of the pathogen.