A mutation conferring plasmid establishment deficiency in Escherichia coli K-12 strain JF55 has been located on the genetic linkage map. This mutation maps at approximately min 48 and is contransducible with purF. A new use of the process of zygotic induction has been explored to indirectly test the ability of an Est⁻ strain to support the first stage in plasmid establishment. According to zygotic induction evidence employing isogenic strains differing only in the est locus mapped, this mutation influences a stage of plasmid establishment later than the conversion of the single-stranded plasmid DNA to the double-stranded form. In addition, evidence is presented that there is more than one gene influencing plasmid establishment in strain JFSS. One such gene is lon. Other mutations, possibly mapping near his and pyrF may influence the first stage of plasmid establishment in this strain. / Master of Science