The Chinese Hamster Ovarian cells CHO-K1 are one of the most extensively used cells for the evaluation of gene expression and toxicology. However, these cells are frequently used for biomedical research without consideration of their cytogenetic characteristics. Therefore, we carried out to investigate the karyologic profiles, the frequency and type of chromosome aberration, and the distribution of telomeric DNA on chromosomes of the CHO-K1 cells. The GTG banding and fluorescence in situ hybridization on CHO-K1 cells were performed to characterize the karyotype and the distribution of telomeric DNA The present study revealed that the chromosome modal number of CHO-K1 cells was 2n=20; eight chromosomes appeared to be identical with those of the normal Chinese hamster, whereas the remaining 12 chromosomes were shown to be translocated, deleted, inversed, or rearranged from Chinese hamster chromosomes. The telomeric DNA on CHO-K1 chromosomes was intensively distributed at the centromeres rather than the ends of chromosomes. In addition, three chromosomes had interstitial telomeres and one marker chromosome entirely consisted of telomeric DNAs. The frequency and type of chromosome aberrations in CHO-K1 cells were examined. Of the 822 metaphase spreads, 68 (8.3%) cells resulted in chromosome aberrations of which the chromosome breakage was the most frequently occurred.