The cost of antimicrobial resistance (AMR) is the reduction of fitness of a resistant mutant relative to a susceptible strain in the absence of drug. Costs of resistance are usually estimated in a single environment and on one genetic background; these fitness estimates may not be representative of what happens in nature. I measured the fitness of AMR E. coli strains in different environments, including medically and ecologically relevant ones. To do this, a collection of AMR strains of Escherichia coli bearing a single resistance mutation were competed against their ancestral strain in 10 different media. The results of this study indicate that laboratory media does not predict fitness in natural environments. We found environments in which resistance alleles suffered no cost, suggesting that these mutants may persist for long periods of time. Data on the fitness of AMR pathogens across environments will help manage their spread.