Salmonella typhimurium strains containing a duplication of nearly a third of the genome have been isolated by a simple procedure involving selection for improved utilization of L-malate as sole carbon source. The duplication occurs at a very high spontaneous frequency. Strains containing the duplication can be isolated selectively on malate medium, or by a non-selective procedure involving Hfr conjugation. When strains containing the duplication are maintained on non-selective medium, the duplication is readily lost. Genetic evidence suggests that the duplication is chromosomal and tandem. The fact that the recA gene is included in the duplication has been used to obtain evidence that the recA1 marker is recessive to its wild-type allele. Unlike tandem duplications previously described in I. coli, the duplication described in this report appears to have unique endpoints.