A new population model is proposed for the growth of Saccharomyces cerevisiae and thereafter identified during synchronized continuous cultures where the initiation of cell division can be easily monitored from on-line measurements. The new model predicts the evolution of the cell number for different sub-populations: adult cells consisting of budding mother cells and adult cells waiting in G1 phase; daughter cells consisting of buds attached to mother cells in G2 or M phase, expanding newborn cells detached from the mother cells and daughter cells from the former generation. Experimental data support the hypothesis that only a fraction p of adult cells will enter mitosis. The lower the proportion of mother cells, the shorter the oscillation period. A segregated model is proposed to couple the population model with the specific expansion rate of the new cells. The variation in biomass formation rate observed in synchronized cultures can be satisfactorily predicted using a first-order kinetic expression with a time constant of about 0.9 h. The model shows that the population is alternatively adapting to a high rate of expansion of new cells and adapting to a high catabolic activity of the old cells when the number of expanding cells is low. This results in a low biomass formation rate and a high CO2 production rate when the mother cells are budding.