The work presented here arose from studies on the regeneration of livers of rats given carcinogens. The scant literature on the distribution of mitotic figures in regenerating liver of normal rats led one to expect a random (Poisson) distribution in the controls. However a non-random distribution of mitotic figures was found in the control animals. This somewhat unexpected finding led to a fuller investigation of the mitotic distribution in “normal” rats after partial hepatectomy. A non-random distribution was found in practically all rats, irrespective of age, diet or laboratory of origin. This distribution corresponded very closely to a Neyman Type A (contagious) distribution, and is characteristic of a random distribution of clusters and a superimposed random distribution within clusters. This is typical of what would be expected for clonal selection which is apparently proceeding throughout life in all so-called normal rats.