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A temperature-sensitive DNA replication mutant of Staphylococcus aureus NCTC 8325 has been isolated and characterized. After transfer to the non-permissive temperature (42 °C), DNA synthesis continued for 30 min and the mean DNA content increased by 56%. The amount of residual DNA synthesis was not reduced when the non-permissive temperature was raised, nor when chloramphenicol was added at the time of the temperature shift. During incubation at 42 °C, mutant bacteria accumulated the capacity to synthesize DNA after return to the permissive temperature (30 °C) in the presence of chloramphenicol. This capacity was lost when chloramphenicol was present at 42 °C. The properties of the mutant are consistent with a defect in the initiation of DNA replication at 42 °C.