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It has been reported that Sinorhizobium meliloti strains harboring IncQ and IncP multicopy vectors containing constitutively expressed Klebsiella pneumoniae nifA exhibit an increase in nodulation competitiveness on alfalfa (J. Sanjuan and J. Olivares, Mol. Plant-Microbe Interact. 4:365–369, 1991). In our efforts to understand the mechanisms involved, in this work, we have found that the observed enhancement on nodulation competitiveness by IncQ derivatives carrying K. pneumoniae nifA was not dependent on the plasmid-borne nifA activity but on the sensitivity of nonresistant strains to the streptomycin carried over from growth cultures. Furthermore, it was also determined that the nifA of K. pneumoniae on an IncP vector does not have an effect on competitiveness.