A total of 476 strains of Escherichia coli isolated from humans, pigs, cattle, poultry, potable water, or effluent were examined for iron-suppressible ability to produce hydroxamate. Isolates able to produce such material (Hyd+ isolates) are presumed to be able to carry out hydroxamate-dependent transport of iron. The percentages of Hyd+ isolates found among E. coli isolated from the feces of breast-fed babies (71%), adults (46%), milk-fed calves (32%), or poultry (28%) were significantly greater (P less than 0.01) than the percentages isolated from potable water and effluent (6%) or from the feces of suckling piglets (6%), weaned pigs (6%), or weaned cattle (4%). The percentages of Hyd+ isolates found among E. coli associated with diarrhea in humans (51%), weaned pigs (7%) or calves (25%) were not significantly different (P greater than 0.1) from those found among strains isolated from corresponding nondiarrheic hosts. Many of the E. coli isolated from cases of E. coli bacteremia in humans and poultry were Hyd+ (64% and 83%, respectively). We conclude that ability to carry out hydroxamate-mediated transport of iron is widely distributed among natural isolates of E. coli but that the distribution of Hyd+ E. coli is not random. E. coli isolated from sources where levels of available iron might be expected to be low tend to be Hyd+. It seems that a link may exist between prevalence of Hyd+ E. coli and active host-defense based on restricted availability of iron.