We have characterized the in vivo phenotypes of 17 mutations of Escherichia coli ftsZ. In particular, we determined whether these mutations can complement a null ftsZ phenotype, and we demonstrated that two noncomplementing mutations show partial dominant-negative behavior. We performed immunofluorescence microscopy to determine whether these mutants could assemble into normal or abnormal structures in vivo. The mutants separated into four classes-those that complemented the null and formed normal FtsZ rings, those that complemented the null but formed aberrant FtsZ structures, those that formed aberrant FtsZ structures and did not complement, and those that were unable to form any FtsZ structures. We did not find any mutations that produced nonfunctional Z rings of normal appearance. Surprisingly, some mutants that produced extensively spiraled Z-ring structures divided and grew with a normal doubling time. The analysis was carried out using a complementation system based on an ftsZ deletion strain, a temperature-sensitive rescue plasmid, and a complementation vector that placed mutated ftsZ alleles under the control of the pBAD promoter, which offered several advantages over previous systems.