Growth advantage in stationary phase (GASP) is the term used to describe the ability of mutants with an increased fitness from 10-day-old enterobacteria culture to out-compete 1-day-old cells of the same initial strain during a prolonged stationary phase, although the aged cells are introduced as a minority. We studied this bacterial trait in mixed cultures of two enterobacterial species, Escherichia coli and Salmonella enterica, wild type in addition to derived mutants from both strains that contain chromosomal-encoded resistance to either nalidixic acid or streptomycin. The strong GASP phenotype was obtained in mixed cultures with the aged mutant strains, but not when the isogenic antibiotic-sensitive strains were used. This phenomenon was associated with chromosomal rearrangements in 10-day-old bacterial antibiotic-resistant mutated cells.