Tomato seedlings growing aseptically in Murashige and Skoog Medium were inoculated with Azorhizobium caulinodans ORS571 (pXLGD4), carrying the lacZ reporter gene. By microscopic analyses of inoculated tomato roots, it has been demonstrated that the xylem of tomato roots can be colonized by Azorhizobium. We discuss whether this colonization of the xylem of tomato roots by diazotrophic azorhizobia might provide a suitable niche for endophytic nitrogen fixation.