Summary: strain 4 a is temperature-sensitive. It grows normally at 25° but stops dividing immediately when shifted to 42°. After the shift cell mass increases at a normal rate and DNA synthesis is unaffected. Viable count remains constant for about 3 h. and then falls off sharply. The filaments formed at 42° divide when returned to 25° after a delay which in broth is about 70 min. This delay is not markedly dependent on the time previously spent at 42° but is mediumdependent, being shorter in enriched media. Eventually all the divisions prevented at 42° take place at 25°. Chloramphenicol almost completely stops filament septation and division, and when added early in the recovery period causes fragility, some lysis and a sharp fall in viable count. Nalidixic acid does not stop filament division. It is concluded that a shift to 42° influences cell division directly by causing the irreversible inactivation of a component required at a late stage of division. The length of the division recovery period when filaments are returned to 25° suggests that the component must accumulate throughout most of the division cycle before division can restart.


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