Isolates of Macrophomina phaseolina were classified into groups by cluster analysis based on their antigenic composition; however, grouping the isolates was not related to their virulence, geographic origin, or host. Cotton cultivar Giza 75 and the most pathogenic isolates from cotton were placed in a separate cluster based on their antigenic composition. The relationship between common antigens, shared by Giza 75 and cotton isolates, and pathogenicity of these isolates was quantified by correlation and regression analyses. There was a positive significant correlation (r = 0.91, p < 0.05) between simple matching coefficient (SSM), established between M. phaseolina isolates and Giza 75, and pathogenicity of the isolates on this cultivar. The regression model indicated that SSM accounted for 84% of the total variation in pathogenicity of the isolates. These results imply that the common antigenic determinants shared by cotton and M. phaseolina isolates are related to severity of charcoal rot.