The average diameters of chromosomes in nuclei of Gyrodinium sp. clearly increased from 0.47 μm at late anaphase to 1.81 μm at last interphase. Electron microscopic autoradiography was used, after the application of thymidine-³H to the resting cells of this dinoflagellate, and DNA synthetic sites were found to be limited to the margins of these chromosomes. Ultrastructures of these sites revealed that there are no membranous structures, but numerous paired granular strings connecting the bases of chromonemata with the nucleogranules. Thus, it was interpreted that the increase in chromosome diameter during the period from the late anaphase to the last interphase of Gyrodinium sp. was attributable to the incorporation of nucleogranules, which accompany DNA synthesis, into the bases of chromonemata coiling in these chromosomes.