A simple method, using commercial disks to detect Escherichia coli mutator strains, is proposed. The breakpoint for detecting strains with a mutation frequency >/=5 x 10(-7) was established at >/=70 and >/=20 colonies in the inhibition zone of fosfomycin and rifampin disks, respectively, after seeding 100 microl of an overnight culture. Strains with <30 and <10 colonies in fosfomycin and rifampin inhibition zones are presumptively non-mutators.