The ftsZ gene is essential for initiation of cell division in Escherichia coli and Bacillus subtilis. To begin our studies of division arrest during differentiation of Rhizobium meliloti bacteroids, we isolated a R. meliloti ftsZ homolog, ftsZ(Rm). Degenerate primers directed towards a conserved region of ftsZ were used to amplify a segment of R. meliloti DNA by polymerase chain reaction, and the product of this reaction was then used to isolate positive clones from a bacteriophage library. The DNA sequence of an open reading frame containing the region of homology indicated that the R. meliloti FtsZ protein (FtsZ(Rm)) is 50% homologous to the known E. coli and B. subtilis FtsZ proteins, but at 590 amino acids (63 kDa), it is predicted to be nearly 50% larger. Strong expression of an approximately 70-kDa labeled protein in a coupled in vitro transcription-translation system supports this prediction. The additional 200 amino acids appear to fall in a single internal domain highly enriched for proline and glutamine residues. When we regulated R. meliloti ftsZ (ftsZ(Rm)) expression on a high-copy-number plasmid in E. coli with P(lac) and lacI(q), cells were smaller than normal in the presence of low FtsZ(Rm) levels (with no isopropyl-β-D-thiogalactopyranoside [IPTG]) and filamentous when FtsZ(Rm) was overproduced (with IPTG). These results suggest that low levels of FtsZ(Rm) stimulate E. coli cell division, while high levels may be inhibitory.