The fine structure of constitutive heterochromatin and euchromatin was compared in electron microscope whole-mount preparations of Tenebrio molitor (Insecta, Coleoptera) spermatocyte nuclei. Tenebrio molitor pachytene chromosomes display extended segments of centromeric heterochromatin and thus are especially suitable for this purpose. When nuclei were incubated in solutions containing different concentrations of NaCl or of MgCl2, two levels of chromatin fine structures were observed in the euchromatic segments: nucleosome fibers (0.1 mM-20 mM NaCl) and supranucleosomal fibers with 28 nm in diameter (40 mM-100 mM NaCl, 0.2 mM-1.0 mM MgCl2). The fine structure in the heterochromatic segments was the same as that in the euchromatic segments in all NaCl concentrations and in MgCl2 concentrations up to 0.4 mM. In higher MgCl2 concentrations the heterochromatin remained more compact than the euchromatin and consisted of 37-nm-thick fibers in 0.6 mM MgCl2 and of 65-nm-thick fibers in 1.0 mM MgCl2. After the 37-nm and the 65-nm fibers had been dispersed in Mg2+-free solutions they could be recondensed by incubation in 0.6 mM and 1.0 mM MgCl2, respectively. It is concluded that a Mg2+-sensitive component of the heterochromatin is responsible for the folding of the nucleosome chain to heterochromatin-specific supranucleosomal structures.