Iron is indispensable to the growth and metabolism of all marine organisms, including bacteria. In this work, we investigated and compared the influence of iron(III) concentration on the growth of and siderophore production by two heterotrophic bacteria--Micrococcus luteus and Bacillus silvestris. Our results showed that the iron concentration strongly influences the growth of both species. The growth curves were different for each iron concentration and each strain. M. luteus grew more rapidly than B. silvestris, but produced a roughly four times smaller quantity of siderophores. Both M. luteus and B. silvestris secreted hydroxamate-type siderophores and alpha-keto/alpha-hydroxy acids, but did not produce catecholates. This paper is probably the first to report on siderophore production by B. silvestris and M. luteus isolated from seawater. Moreover, the influence of different iron concentrations on the growth of and siderophore production in these bacteria has been documented. This provides further evidence indicating iron bioavailability as the actual reason for siderophore release by biota.