-Chromosome number, interphase nuclear structure, prophase chromosome condensation patterns, and meiotic behaviour were analysed in 14 Brazilian species within the families Loranthaceae and Viscaceae. All the species showed reticulate interphase nuclei and an uniform pattern of prophase chromosome condensation. The eleven species of Loranthaceae studied had 2n=16, whereas the three species of Viscaceae (Phoradendron) had 2n=28. The mitotic chromosome of only two species of Phoradendron revealed a significant karyotype asymmetry, while the remaining karyotypes were more symmetrical, and similar to each other. By contrast, the meiotic behaviour of ten species analysed exhibited several meiotic irregularities, characteristics of structural heterozygosis. In one sample of Struthanthus syringifolius, the majority of the meiocytes showed a ring tetravalent. The stability in mitotic karyotype associated with the meiotic irregularities observed in several genera of these families suggest that in spite of the structural changes orthoselection has preserved a diploid basal karyotype with large and symmetrical chromosomes. It is hypothesized that this karyotype was established before the diversification of these families and its large DNA amount has hindered the occurrence of polyploidy in these families.