Most mutations are likely to be deleterious, and so the spontaneous mutation rate is generally held at a very low value(1). Nonetheless, evolutionary theory predicts that high mutation rates can evolve under certain circumstances(2-4). Empirical observations have previously been limited to short-term studies of the fates of mutator strains deliberately introduced into laboratory populations of Escherichia coli(5-7), an to the effects of intense selective events on mutator frequencies in E. coli(8). Here we report the rise of spontaneously originated mutators in populations of E. coli undergoing long-term adaptation to a new environment; Our results corroborate computer simulations of mutator evolution in adapting clonal populations(4), and may help to explain observations that associate high mutation rates with emerging pathogens’ and with certain cancers(10).