Abstract The production of aerobactin was studied in 943 clinical isolates of 14 different Gram-negative genera. Aerobactin-producing strains were isolated in all the genera of Enterobacteriaceae tested, whereas no aerobactin-producing strains were found in the non-enterobacterial genera ( Aeromonas, Pseudomonas, Acinetobacter). Enterobacteriaceae might be divided in 2 groups. The first with high incidence of aerobactin-producing strains (more than 40%) comprised the genera of Enterobacter, Shigella and Escherichia. The second, with low incidence of aerobactin-producing strains (less than 20%) comprised the genera Klebsiella, Citrobacter, Proteus, Morganella, Yersinia, Serratia and Salmonella. The production of the siderophore is described for the first time in Citrobacter and Hafnia.