A temperature-sensitive mutation (T46) affecting the initiation of DNA replication in Escherichia coli is not complemented by an F factor. If, however, temperatureresistant revertants are isolated from a T46 s F + strain most of them are Hfr, with F integrated at various sites. These Hfr temperature resistant cells still contain the T46 s mutation. They are sensitive to acridine orange and ethidium bromide at 42 °C, but not at 30 °C, suggesting that, at the restrictive temperature, they are under the control of the F replicon. Similar results were obtained in a partial diploid strain carrying the F′ lac factor. In a large number of the Hfr temperature resistant strains formed, F′ lac had integrated in the lac region of the chromosome, but Hfr temperature resistant strains were also produced by integration of F′ lac at other sites. “Integrative suppression” by F′ lac was also observed for another temperaturesensitive mutation affecting the initiation of DNA replication (T83). It requires the function specified by the recA gene. Integrative suppression does not take place for mutations which affect the continuation of DNA replication rather than its initiation. F integration alone is not sufficient for integrative suppression and another step is needed. The nature of that step remains unknown. We conclude that, in the Hfr temperature resistant strains, chromosome replication and cell division are under F control. It seems that in these cells the chromosome has become part of the F replicon.