A new temperature-sensitive mutant of E. coli, defective in cell division, was isolated after selection for tolerance to colicin E2. The mutant strain, ASHI24, growing in either minimal or complex medium, commences filament formation immediately upon shift to high temperature. High densities of bacteria or the presence of 0-44 M-sucrose prevents filament formation at 42 degrees C and division continues. Filament formation in the mutant is reversible and upon return to 29 degrees C the multinucleate filaments divide up into normal-sized bacteria by a series of rapid but sequential divisions. In the presence of chloramphenicol at 29 degrees C, 25% of these division sites are still expressed. A genetic locus designated ftsH, apparently controlling both temperature sensitivity and filament formation, was provisionally mapped at minute 80 on the E. coli K12 map.