A plate assay to show the production of aerobactin in an agar medium was optimized by using a mutant Escherichia eoli strain as indicator (from V. Braun, T/ibingen). This strain is deficient in producing and transporting enterochelin and in producing aerobactin, but able to transport aerobactin. A number of various species of Enterobacteriaceae was tested to check the accuracy of this method. We found that almost all of the strains coding for aerobactin production are bearing plasmids and, in certain cases, were able to localize the aerobactin information on conjugative R-plasmids. This underlines the fact that aerobactin genes are rarely found on the chromosome. The usefulness of the aerobactin assay in the routine laboratory as well as a possible relationship between aerobactin production and virulence is discussed.