Compared with other labeling techniques, the use of the green fluorescent protein (GFP) is advantageous to visualize bacteria because observations can be performed in real time. This feature is particularly interesting to study invasion events of rhizobia during nodule development on their legume host plant. To investigate the symbiotic interaction between Azorhizobium caulinodans ORS571 and Sesbania rostrata, we constructed two plasmids, pMP220-hem-gfp5 and pBBR5-hem-gfp5-S65T, that carry a modified gfp gene, the expression of which is controlled by the constitutive hem promoter. Introduction of either of these plasmids into A. caulinodans allowed the visualization of single bacteria. Determination of the plasmid stability in cultured bacteria and in nodules demonstrated that pBBR5-hem-gfp5-S65T is more stable than pMP220-hem-gfp5. The plasmid pBBR5-hem-gfp5-S65T can be used to study early invasion events during nodule development on hydroponic roots of S. rostrata.