A method is described for calculating the dynamics of the distribution of fitness in an infinite asexual population which is subject to unconditionally deleterious mutations with independent effects. This method is applied to the problem of calculating the frequency of a mutator subpopulation, at equilibrium between mutation and indirect selection due to association with deleterious mutations. Many mutator alleles are produced by loss–of–function mutations in polymerase or mismatch repair genes. Previous calculations have ignored the fact that this creates a flux of higher fitness individuals into the mutator subpopulation. This flux raises the mean fitness of the mutator subpopulation, and when this factor is taken into account, the frequency of the mutator may be more than an order of magnitude greater than recent theoretical work has suggested.