The mitotic behaviour of diplochromosomes was investigated in early calli from leaf segments of a monohaploid potato (x = 12) with the aid of the fluorescent plus Giemsa technique. The sister chromosomes of the diplochromosomes do not separate during metaphase because the inner chromatids remain connected. Consequently the diplochromosomes orient in three stable configurations during metaphase. In the case in which sister chromosomes become auto-oriented, the outer chromatids face one pole and the inner chromatids the opposite pole (U-shaped configuration), or the outer and inner chromatid of each sister chromosome face different poles (alternate configuration). Less frequently the sister chromosomes become oriented to different poles (C-shaped configuration). Linear orientation is not observed. At least until the onset of telophase only the inner chromatids originating from U-shaped configurations may remain paired, but they fall apart before the next metaphase. The mitoses with diplochromosomes give rise to two cells with double the number of chromosomes.Key words: potato, diplochromosomes, mitosis, orientation, polyploidization.