The essential cell division protein FtsZ forms a dynamic ring structure at the future division site. This Z‐ring contracts during cell division while maintaining a position at the leading edge of the invaginating septum. Using immunofluorescence microscopy we have characterized two situations in which non‐ring FtsZ structures are formed. In ftsZ26 (temperature sensitive, Ts) mutant cells, FtsZ‐spirals are formed and lead to formation of spirally invaginating septa, which in turn cause morphological abnormalities. In rodAsui mutant cells, which grow as spheres instead of rods, FtsZ‐arcs are formed where asymmetric septal invaginations are initiated. The FtsZ‐arcs later mature into complete FtsZ‐rings. Our data show that Z‐spirals and Z‐arcs can contract and that in doing so, they determine the shape of the invaginating septa. These results also strongly suggest that in normal cell division, FtsZ is positioned to a single nucleation site on the inner membrane, from which it polymerizes bidirectionally around the cell circumference to form the Z‐ring.