Elongation of many rod-shaped bacteria occurs by peptidoglycan synthesis at discrete foci along the sidewall of the cells. However, within the Rhizobiales, there are many budding bacteria, in which new cell growth is constrained to a specific region. The phylogeny of the Rhizobiales indicates that this mode of zonal growth may be ancestral. We demonstrate that the rod-shaped bacterium
Agrobacterium tumefaciens
grows unidirectionally from the new pole generated after cell division and has an atypical peptidoglycan composition. Polar growth occurs under all conditions tested, including when cells are attached to a plant root and under conditions that induce virulence. Finally, we show that polar growth also occurs in the closely related bacteria
Sinorhizobium meliloti
,
Brucella abortus
, and
Ochrobactrum anthropi
. We find that unipolar growth is an ancestral and conserved trait among the Rhizobiales, which includes important mutualists and pathogens of plants and animals.