Highly efficient nitrogen-fixing strains selected in the laboratory often fail to increase legume production in agricultural soils containing indigenous rhizobial populations because they cannot compete against these populations for nodule formation. We have previously demonstrated, with a Sinorhizobium meliloti PutA mutant strain, that proline dehydrogenase activity is required for colonization and therefore for the nodulation efficiency and competitiveness of S. meliloti on alfalfa roots (J. I. Jiménez-Zurdo, P. van Dillewijn, M. J. Soto, M. R. de Felipe, J. Olivares, and N. Toro, Mol. Plant-Microbe Interact. 8:492-498, 1995). In this work, we investigated whether the putA gene could be used as a means of increasing the competitiveness of S. meliloti strains. We produced a construct in which a constitutive promoter was placed 190 nucleotides upstream from the start codon of the putA gene. This resulted in an increase in the basal expression of this gene, with this increase being even greater in the presence of the substrate proline. We found that the presence of multicopy plasmids containing this putA gene construct increased the competitiveness of S. meliloti in microcosm experiments in nonsterile soil planted with alfalfa plants subjected to drought stress only during the first month. We investigated whether this construct also increased the competitiveness of S. meliloti strains under agricultural conditions by using it as the inoculum in a contained field experiment at León, Spain. We found that the frequency of nodule occupancy was higher with inoculum containing the modified putA gene for samples that were analyzed after 34 days but not for samples that were analyzed later.