Response to changes in light conditions involves a variety of receptors that can modulate gene expression, enzyme activity and/or motility. For the study of light-regulated effects of Agrobacterium tumefaciens, we used a global analysis approach - proteomics - and compared the protein patterns of dark- and light-grown bacteria. These analyses revealed a significant reduction of FlaA and FlaB - proteins of the flagellum - when the cells were grown in light. The light effect was confirmed by SDS-PAGE with isolated flagella. Quantitative PCR experiments showed a 10-fold increase of the transcription level of flaA, flaB and flaC within 20 min after the transfer from light to darkness. Electron microscopy revealed that these molecular events result in a light-induced reduction of the number of flagella per cell. These changes have major physiological consequences regarding motility, which is considerably reduced with exposure to light. The inhibitory effect of light on the motility is not unique to A. tumefaciens and was also seen in other species of the Rhizobiaceae. Previous studies suggested that the flagella function is significant for bacteria-plant interactions and bacterial virulence. In our studies, light reduced the attachment of A. tumefaciens to tomato roots and the virulence of the bacteria in a cucumber infection assay.