A hybrid assay, based on the properties of the λ repressor, was developed to detect FtsZ dimerization in Escherichia coli in vivo. A gene fusion comprising the N-terminal end of the λ cl repressor gene and the complete E. coli ftsZ gene was constructed. The fused protein resulted in a functional λ repressor and was able to complement the thermosensitive mutant ftsZ84. Using the same strategy, a series of 10 novel mutants of FtsZ that are unable to dimerize was selected, and a deletion analysis of the protein was carried out. Characterization of these mutants allowed the identification of three separate FtsZ portions: the N-terminal of about 150 amino acids; the C-terminal of about 60 amino acids, which corresponds to the less conserved portion of the protein; and a central region of about 150 residues. Mutants belonging to this region would define the dimerization domain of FtsZ.