A mutant Hfr strain of Escherichia coli which has an impaired incompatibility function but is normal for other F factor functions has been isolated. This Inc(-) Hfr permits the maintenance and transfer of both the integrated F factor and an F' factor. F' factors have been isolated from the integrated F factor of the Inc(-) Hfr strain. When these episomes were tested in matings with Hfr or F' strains, they did not differ in any observed way from wild-type F' factors.