Owing to occasional spontaneous mutations in genes encoding DNA repair, any population of a reasonable size is expected to harbor a sub-population of genetic mutators. Using a genetically modified strain of Escherichia coli K-12, we have estimated the frequency of mutators to be about 3x10(-5). By and large, this corresponds to a mutation rate from non-mutators to mutators of 5x10(-6) per bacterium per generation. Using a mutS∷Tn10 derivative as representative for mutators, we estimated the increase in mutation rates in mutators to be 19- to 82-fold, depending on the test-mutation under consideration. The load associated with this increase in mutation rate resulted in a growth inhibition of 1%. From these data, we estimated that the rate of detrimental mutations in the non-mutators to be 2x10(-4)-8x10(-4). The situations where adaptive mutations may result in an increase in the frequency of mutators are discussed.