The cells of cryptophycean and chlorarachniophycean algae contain a nucleomorph, a vestigial nucleus derived from red and green algal endosymbionts respectively. The origin of the nucleomorph is therefore different from that of cellular organelles such as mitochondria and chloroplasts. In this study, we sought to determine whether cell cycle regulation of the nucleomorph in the cryptophycean alga Pyrenomonas helgolandii is functionally similar to that of the cell nucleus. We performed an ultrastructural analysis of nucleomorph division in cells prepared by rapid freezing fixation - freeze substitution and also carried out BrdU labeling experiments to determine the timing of nucleomorph DNA synthesis in relation to that of the cell nucleus. In cells cultured under 16 hours light: 8 hours dark conditions, BrdU labeling experiments showed that DNA synthesis in the nucleomorph occurred during a limited period from 2 hr to 4 hr after the beginning of the dark period. The S phase in the nucleomorph started just after completion of the nuclear S phase. Thus, DNA synthesis in the nucleomorph occurred at a defined period of the cell cycle. By contrast, our BrdU experiments showed that the nucleoids of mitochondria and chloroplasts could perform DNA synthesis throughout the whole cell cycle.