Karyology and genome size of four species of the genus Onosma from northern border of the area of the genus in Slovakia were studied. Different chromosome number is characteristic for each of the studied representatives. O. tornensis (2n=14) has 7 pairs of homologous S (short) chromosomes 1.62 -2.94 µm long, 5 pairs are metacentric and 2 pairs are submetacentric; nuclear DNA content: 1.56 pg/2C. O. visianii (2n=18) has 9 pairs of homologous chromosomes 1.91 -3.18 µm long, 3 pairs are

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... , 4 pairs submetacentric and 2 pairs are subacrocentric (one pair bears satellites); nuclear DNA content 4.12 pg/2C. O. arenaria (2n=20) has 6 pairs of homologous metacentric L (long) chromosomes 3.22 -4.33 µm long and 8 unpaired S chromosomes 1.07 -2.36 µm long, 7 of these chromosomes are metacentric and 1 submetacentric; nuclear DNA content 5.15 pg/2C. O. pseudoarenaria (2n=26) has 6 pairs of homologous metacentric L chromosomes 3.36 -5.03 µm long and 7 pairs of homologous metacentric S chromosomes 1.07 -2.36 µm long and nuclear DNA content 5.74 pg/2C. Conspicuous difference in chromosome structure of particular species and in their DNA content is further discussed in the paper.