Colonization levels in five tissues after avian pathogenic Escherichia coli (APEC) inoculation were investigated in chickens to generate phenotypic data for a genome wide association study (GWAS). Bacterial loads were measured in 370 birds and varied among individuals and tissues. Mean bacterial levels were significantly different between tissues (right lung > spleen > left lung and liver > blood). There were also significant correlations in bacterial load between tissues. These data suggest that colonization levels could be used as phenotypes in GWAS and could help identify markers associated with poultry resistance to APEC infections. After verification, these markers could be used for genetic selection for more resistant chickens.