We have investigated the initiation of DNA replication in Escherichia coli gyrB (Ts) mutants and find that initiations in single cells, even those that occur at non-permissive temperature, are synchronous. Furthermore, our results indicate that the gradual arrest of DNA replication at non-permissive temperature reflects a general decrease in transcription activity and not an initiation-specific function of the DNA gyrase. At an intermediate temperature, the only effect observed was a lack of segregation (decatenation) of replicated chromosomes in a sizeable fraction of the cell population.