Inhibition studies with free-living cells of Rhizobium meliloti 201 1 showed that succinate, fumarate and malate are transported via a common transport system. It is an active process that is inducible by succinate. The apparent K,,, for succinate uptake in free-living cells is 5.3 p~. Seven Tn.5-induced mutants that did not grow on succinate, fumarate or malate lacked the C4-dicarboxylate transport system in the free-living state (Dctfl-). Five of these mutants (RMS11, RMS16, RMS17, RMS24, RMSl18) induced nodules on alfalfa with about half the nitrogenfixation activity of the wild-type (Fixred). The remaining two mutants (RMS420 and RMS938) gave rise to small, white and ineffective nodules on alfalfa (Fix-). These results correlated with the dicarboxylate transport rates of bacteroids isolated from appropriate nodules : bacteroids isolated from nodules induced by the Fixred mutants transported succinate or malate at about 30-50% of the wild-type rate (Dctsred), whereas bacteroids isolated from nodules induced by Fixmutants showed no uptake activity (Dcts-). From an R. meliloti 201 1 bank three cosmids were identified which complemented different Dctfl-mutants. Complemented Dctfl-mutants could grow again on agar containing succinate or malate. In the case of Fix-mutants complementation resulted in effective nodules. The cosmids pRmSC 101 and pRmSClO2 complemented the Fix-mutant RMS420. A third cosmid, pRmSC121, complemented the second Fix-mutant RMS938 and nearly all of the Fixred mutants. For the Fix-mutant RMS420 the Tn5 insertion site was found to be located on a 15-3 EcoRI fragment which is also part of the complementing cosmid pRmSC102.