Many species of pseudomonads produce fluorescent siderophores involved in iron uptake. We have investigated the DNA homology between the siderophore synthesis genes of an opportunist animal pathogen, Pseudomonas aeruginosa, and three plant-associated species Pseudomonas syringae, Pseudomonas putida and Pseudomonas sp. B10. There is extensive homology between the DNA from the different species, consistent with the suggestion that the different siderophore synthesis genes have evolved from the same ancestral set of genes. The existence of DNA homology allowed us to clone some of the siderophore synthesis genes from P. aeruginosa, and genetic mapping indicates that the cloned DNA lies in a locus previously identified as being involved in siderophore production.