During cell division in the yeast Saccharomyces cerevisiae mother cells produce buds (daughter cells) which are smaller and have longer cell cycles. We performed experiments to compare the lengths of cell cycle phases in mothers and daughters. As anticipated from earlier indirect observations, the longer cell cycle time of daughter cells is accounted for by a longer Gi interval. The S-phase and the G2-phase are of the same duration in mother and daughter cells. An analysis of five isogenic strains shows that cell cycle phase lengths are independent of cell ploidy and mating type.At cytokinesis in the yeast Saccharomyces cerevisiae, mother cells and daughter cells (the buds) are unequal in size. The smaller daughter cells consequently have a longer cell cycle (4,5,8). Most of the increase in cell cycle length in daughter cells occurs in the interval from cell division to emergence of a new bud, and before the step called Start (9; reviewed in reference 11). Since the Start function appears to take place at, or close to, the boundary between the Glphase and the S-phase, it is likely that the differences in cell cycle lengths for mothers and daughters is entirely accounted for by a difference in the Gl-phase (4,5,8). This inference, however, has not been tested directly. In addition, it is not known whether the S-and G2-phases are equivalent in mothers and daughters. The results of a direct determination of cell cycle phase durations are presented in this report.The length of the S-phase in total populations of S. cerevisiae has been estimated for only a few strains. The published data indicate that a diploid genome can be replicated faster than a haploid genome (2,13,14,16). The experiments, however, were carried out under different growth conditions and in strains with different genetic backgrounds. We therefore examined the length of cell cycle phases in an isogenic set of haploid and diploid strains under constant growth conditions. In addition, we determined whether mating type influences cell cycle phase durations.Mother and daughter cells can be distinguished microscopically by a ring of chitin, the bud scar, that forms on the cell surface of the mother at the site of emergence of each bud (1). We coupled the fluorescent staining of bud scars with whole cell autoradiography to determine cycle phase lengths of mother and daughter cells. A culture of the aa diploid A364A D5 (7) growing in glucose minimal medium (13,18) (Fig. 1). Budded cells undergoing nuclear division (Fig. 1, arrow) were included in the budded class. The budded cells with two nuclei were interpreted to be cells in which the nuclei, having completed division, had reentered the Gl-phase, but the cells either had not completed cytokinesis or had failed to be separated by sonication. Therefore, the completion of nuclear division is taken as the boundary between the G2-phase and the Gl-phase. In our analysis, both mother and daughter Gl doublets (ca. 10% of the sample) contribute two cells to the unbudded classes: in each doublet one cell is a mo...