We tagged Agrobacterium tumefaciens cells with a mini-Tn5 transposon containing a promoterless gene encoding a green fluorescent protein (GFP). Some of the GFP-tagged individual bacterial cells exhibited strong green fluorescence, which reflected the expression levels of the GFP-tagged genes. Those cells could be readily detected with a confocal laser scanning microscope (CLSM). We observed that the fluorescence and morphology of A. tumefaciens cells grown in plant tissues resembled those grown in a minimal medium of low pH, which is required for expression of the virulence genes responsible for tumorigenesis. This suggests that GFP-aided CLSM can be used to determine which growth medium is more representative of the nutritional conditions that a pathogen encounters in plant tissues. We also observed that the fluorescence and morphology of A. tumefaciens cells changed dramatically during the course of infection. Our data suggested that A. tumefaciens cells were probably better fed upon successful colonization. We believe that GFP-aided CLSM can help study the fate of A. tumefaciens cells inside plant tissues by monitoring cell morphology and gene expression associated with the infection process in situ.