Rhizobium sp. strain BICC 651 in the presence of 100 mu M Al3+ produced a threefold higher level of siderophore than in the control culture under iron limitation during the stationary phase. Al3+ in increasing concentrations resulted in decreased growth, and the effect was alleviated by the addition of iron. Siderophore production decreased gradually in Al3+-treated culture as well as in the control with the addition of increasing concentrations of Fe3+, and at 50 mu M Fe3+ the level of siderophore was practically undetectable. The siderophore binds Fe3+ and also Al3+. The outer membrane protein profiles of the bacteria grown in the presence or absence of Al3+ were indistinguishable.