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Cotyledons of resistant and susceptible cotton (Gossypium hirsutum) cultivars were inoculated artificially with four isolates of Xanthomonas campestris pv. malvacearum from Africa (HV1, HV3, HV7 and Sudan) and USA race 18. Each isolate was used as single inoculum and in combinations of mixed inoculum. Disease reactions were graded 15 days after inoculation on a scale of 1 (immunity) to 10 (fully susceptible). Inoculation with HV1 atone produced the highest disease grade of all inoculum treatments. When HV1 was included in a mixed inoculum, disease grades were greatly reduced compared to HV1 alone. Disease grades were highest for the inoculum mixture of the most virulent isolates HV1 and race 18, but were still reduced compared to those from HVI alone. The efficiency of selection when screening cotton germplasm for bacterial blight resistance may be reduced if the highly virulent HV1 isolate is included in an inoculum mixture composed of non-virulent isolates. [ABSTRACT FROM AUTHOR]