Eight independent temperature-sensitive mutants of the cell division protein FtsA have been studied. They fall into two classes in terms of their behaviour at 42°C and recovery at 30"C. The first class shows saltdependent temperature-sensitivity and reversible inactivation of FtsA protein at 42''C. The second shows irreversible inactivation which is not prevented by salt. Recovery of the ability to divide at 30"C is rapid in mutants of the first group, but is delayed for approximately a generation time in the second group. This suggests that irreversible inactivation of FtsA causes extensive damage to the division machinery. The amino acid substitutions show clustering to a limited domain of the protein, and one particular substitution is found in three of the mutants.