A gene duplication occurs in Salmonella typhimurium which involves about 30% of the genome. The frequency of formation and loss of this duplication was determined in a Rec.A⁻ strain and a PolA⁻ strain and compared to that of a RecA⁺, PolA⁺ strain. The frequency of formation was found to be reduced in both RecA⁻ and PolA⁻. strains. Loss of the duplication was eliminated in the RecA⁻ strain, while the frequency of loss was found to be greater in a PolA⁻ strain than in the PolA⁺ strain. The effect of UV irradiation on the formation of duplication was also compared for the three strains. There is an increase in the frequency of formation in both the PolA⁺, RecA⁺ and PolA⁻, RecA⁺ strains, the increase being greater in the PolA⁻ strain. The frequency of formation is reduced in the RecA⁻ strain. It is postulated that recA-dependent repair pathways are involved in the formation of the duplication, but there is at least one other pathway available. / Master of Science