Native chromosomes from the primitive dinoflagellate Prorocentrum micans were isolated without recourse to chemicals from purified nuclei frozen in liquid nitrogen. Thawing of the nuclear suspension resulted in the disruption of nuclear membrane, and the released chromosomes were incubated in various conditions or directly observed by electron microscopy after spreading. (1) Observations of whole mounted chromosomes confirmed that they were organized in a double-helical bundle of chromosomal fibers. (2) Investigations of these high-ordered chromosomal fibers at high magnification revealed helical compaction of nucleofilaments in a hierarchy of six organization levels (from the DNA to the double helical bundle forming the chromosome). (3) This peculiar architecture was shown to be stabilized mainly by divalent cations; the behavior of native chromosomes incubated in a low concentration of either EDTA or EGTA strongly suggested that the chromosome has two different cation sites, one for Ca2+ and the other for Mg2+.