A spontaneous change in DNA content of a human colonic carcinoma grown in nude mice was observed fortuitously. The tumor initially had a G1 cell DNA content of 1.3 times that of normal cells. Flow cytometric DNA analysis showed in transplant generation 56 the appearance of a new subpopulation which in three passages completely overgrew the original population. The DNA content of the new subpopulation was twice that of the original population. The observation supports the hypothesis of clonal evolution of tumor cell populations. The growth rates of the tumor before and after the change showed no significant difference (p greater than 0.05). Cell kinetic factors, therefore, offer no obvious explanation of how the overgrowth took place. It is not known whether the original population disappeared completely or survived as a small population below the detection limit. The heterogeneity created by clonal evolution of a tumor would be less pronounced if old subpopulations often become extinct as new ones emerge. Heterogeneity of human tumors is of clinical importance because the individual subpopulations may have different sensitivity patterns to antineoplastic drugs.