The cell division protein FtsZ is composed of three regions based on sequence similarity: a highly conserved N-terminal region of approximately 320 amino acids; a variable spacer region; and a conserved C-terminal region of eight amino acids. We show that FtsZ mutants missing different C-terminal fragments have dominant lethal effects because they block cell division in Caulobacter crescentus by two different mechanisms. Removal of the C-terminal conserved region, the linker, and 40 amino acids from the end of the N-terminal conserved region (FtsZdeltaC281) prevents the localization or the polymerization of FtsZ. Because two-hybrid analysis indicates that FtsZdeltaC281 does not interact with FtsZ, we hypothesize that FtsZdeltaC281 blocks cell division by competing with a factor required for FtsZ localization or that it titrates a factor required for the stability of the FtsZ ring. The removal of 24 amino acids from the C-terminus of FtsZ (FtsZdeltaC485) causes a punctate pattern of FtsZ localization and affects its interaction with FtsA. This suggests that the conserved C-terminal region of FtsZ is required for proper polymerization of FtsZ in Caulobacter and for its interaction with FtsA.