We have studied the evolution of the karyotype of 6 tetraploid strains ofChinese hamsterinduced with colchicine. All six strains have lost with time a certain number of chromosomes, and each one of them has become stabilised on an average number, characteristic for each strain. The analysis of the karyotype of these six strains showed that different types of chromosomes behave in different ways; in particular a decrease was observed in the large and mediummchromosomes and in thestchromosomes; thetchromosomes, the smallmchromosomes and the minute chromosomes either remained unchanged with time or increased. It was also observed that while the large and mediummchromosomes are generally in a triploid number the other vary considerably in number from cell to cell. This causes a large variability of the karyotype of different cell populations. In these populations we could not observe the presence of a « stemline » in the classical meaning of this term.