SUMMARYEscherichia coli strains isolated from children with diarrhea were studied to determine the incidence of the components of the aerobactin system. The production of aerobactin and the presence of the aerobactin receptor were demonstrated using bioassay techniques. The presence and location of the genes for the aerobactin system was determined by colony and Southern DNA-DNA hybridization. It was found that aerobactin and the aerobactin receptor could be coded by either chromosomal or plasmid genes. A high percentage of strains expressed the receptor in the absence of aerobactin. Of 19 enterotoxin-producing strains one was found to have the complete aerobactin system, while 11 expressed only the aerobactin receptor which was most frequently coded by a plasmid.