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Mutator alleles, which confer increased mutation rates, are known to spontaneously emerge and "hitchhike" to fixation in evolving asexual populations. Theory predicts that in an evolving asexual mutator population, a second mutator allele may spontaneously arise and hitchhike to fixation. Here, we describe an empirical test of the hypothesis of repeated hitchhiking. The starting population was a clonal strain of mutL − Escherichia coli whose mutation rate was 100-fold higher than wild type. We exposed the mutL − strain to a series of three antibiotics in increasing order of selective strength: fosfomycin, rifampicin, and streptomycin. Two independent replicates of the experiment were performed. As predicted, elevated mutation rates and enrichment for multilocus mutators (which bear more than one mutator allele) were observed in the end point populations of both experiments. DNA sequencing revealed an identical spontaneous 1-bp insertion in the mutator gene mutT in both end point populations. In the multilocus mutators, the causal relationship between the mutT − mutations and the increase in mutation rate was supported with mutT + plasmid complementation tests. Surprisingly, when the experiment was repeated with the antibiotics deployed in decreasing order of selective strength, enrichment for multilocus mutators was not observed. Our data support the likelihood that the mutT − mutations rose to fixation in both populations, consistent with the hypothesis of repeated mutator hitchhiking. The escalation of mutation rates in asexual populations is relevant to multiple biological scenarios, including antibiotic resistance, host–pathogen interactions, and carcinogenesis. [ABSTRACT FROM AUTHOR]