The chromatin Ultrastructure of nuclei in ultrathin sections of root meristem in <i>Cucurbita pepo</i> - the species poor in DNA, and in <i>Haemanthus katharinae</i> - rich in DNA was compared. Four combinations of fixation and embedding were used: 1) OsO<sub>4</sub> (pH 7.2), methacrylates; 2) OsO<sub>4</sub> (pH 7.2), Epon; 3) OsO<sub>4</sub> (pH 6.8), Epon; 4) glutaraldehyde (pH 7.2-7.4) with OsO4 postfixation, Epon embedding. Most suitable was OsO<sub>4</sub> fixation (pH 7.2) and embedding in Epon because of the smallest dispersion of dimensions of chromatin fibrils and good visibility of their substructure. Fibrils of condensed chromatin, components of chromocenters (heterochromatin) in <i>Cucurbita pepo</i>, bands of chromonemata visible in the light microscope and metaphase chromosomes in <i>Haemanthus katharinae</i> showed a diameter of about 11.5 nm after OsO<sub>4</sub> fixation, or 14-19 nm after glutaraldehyde. Fibrils of extended chromatin (euchromatin) in <i>Cucurbita pepo</i> are thinner, their diameter is about 10 nm after OsO<sub>4</sub> fixation and Epon embedding, and about 14.5 nm after glutaraldehyde. There are no visible differences in the width of elementary fibrils after OsO<sub>4</sub> fixation and embedding in methacrylates. In both species the fibrils of extended chromatin are built up of one coiled deoxyribonucleohistone (DNH) thread about 2.3 nm in diameter, while the fibrils of condensed chromatin consist of two DNH threads, each about 3 nm in diameter. The diameter of DNH threads is determined neither by the fixative nor by the way of embedding; they are less visible in the material embedded in methacrylates. The results obtained show that the structure of chromatin fibrils depends on the kind of chromatin, but not on DNA content. In <i>Haemanthus katharinae</i>, the species rich in DNA, the greater amount of chromatin appears in condensed form.