Cultures of recombination-deficient strains of Escherichia coli are composed of three classes of cells: (i) viable cells, which can undergo 20 or more generations, (ii) residually dividing cells, which can undergo fewer than 20 generations (probably an average of fewer than 6), and (iii) nondividing cells, which are incapable of a single division. The nonviable but residually dividing cells contribute to the mass increase of the culture, but not to the viability, thus accounting for the apparent dissimilarity in the effects of rec mutations on growth rates and viabilities. We have determined the frequencies of cells in each of the three classes, and, by making a simplifying assumption concerning the relative division times of viable and residually dividing cells, we have been able to describe mathematically the growth of the rec(-) cultures.