FtsZ and FtsA are essential for cell division in Escherichia coli and colocalize to the septal ring. One approach to determine what regions of FtsA and FtsZ are important for their interaction is to identify in vivo interactions between FtsA and FtsZ from different species. As a first step, the ftsA genes of Rhizobium meliloti and Agrobacterium tumefaciens were isolated and characterized. In addition, an FtsZ homolog that shared the unusual C-terminal extension of R. meliloti FtsZ1 was found in A. tumefaciens. In order to visualize their localization in cells, we tagged these proteins with green fluorescent protein (GFP). When R. meliloti FtsZ1-GFP or A. tumefaciens FtsZ-GFP was expressed at low levels in E. coli, they specifically localized only to the E. coli FtsZ ring, possibly by coassembly. When A. tumefaciens FtsA-GFP or R. meliloti FtsA-GFP was expressed in E. coli, they failed to localize detectably to the E. coli FtsZ ring. However, when R. meliloti FtsZ1 was coexpressed with them, fluorescence localized to a band at the midcell division site, strongly suggesting that FtsA from either A. tumefaciens or R. meliloti can bind directly to its cognate FtsZ. As expected, GFP-tagged FtsZ1 and FtsA from either R. meliloti or A. tumefaciens localized to the division site in A. tumefaciens cells. Therefore, the 61 amino acid changes between A. tumefaciens FtsA and R. meliloti FtsA do not prevent their direct interaction with FtsZ1 from either species, suggesting that those residues are not essential for protein-protein contacts. Moreover, the failure of the two non-E. coli FtsA derivatives to interact strongly with E. coli FtsZ in this in vivo system unless their cognate FtsZ was also present suggests that FtsA-FtsZ interactions have coevolved and that the residues which differ between the E. coli proteins and those of the two other species may be important for specific interactions.