SUMMARYUsing TEM technique for studying the fine structure of dinoflagellate chromosomes in Vrorocentrum micans, we have obtained data supporting the model of stacked series of nested-transversal DNA arcs, held together by a presumable ribonucleoprotein matrix and representing a higher order of organization of a single, or few, very long DNA fibrils folded in a spiral manner around a virtual axis. Both interchromosomal connections and the attachment of chromosomes to the nuclear envelope suggest a superchromosomal organization of the mesokaryote genome, well evidenced when the chromosomes are in a despiralized, relaxed state. We have also obtained proof for the existence of several nucleolar-organizing chromosomes in the same mesokaryote nucleus, all being involved in the organization of the same nucleolus. At the level of nucleolus-organizer region of such NO-chromosomes, DNA fibrils are extensively unfolded, a situation commonly encountered in typical eukaryotes.