Abstract: As yeast cultures enter stationary phase in rich, glucose-based medium, differentiation of two major subpopulations of cells, termed quiescent and non-quiescent, is observed. Differences in mRNA abundance between exponentially growing and stationary-phase cultures and quiescent and non-quiescent cells are known, but little was known about protein abundance in these cells. To measure protein abundance in exponential and stationary-phase cultures, the yeast GFP-fusion library (4159 strains) was examined during exponential and stationary-phases, using high-throughput flow cytometry (HyperCyt(R)). Approximately 5% of proteins in the library showed 2-fold or greater changes in median fluorescence intensity (abundance) between the two conditions. We examined 38 strains exhibiting two distinct fluorescence-intensity peaks in stationary phase and determined that the two fluorescence peaks distinguished quiescent and non-quiescent cells, the two major subpopulations of cells in stationary-phase cultures. GFP-fusion proteins in this group were more abundant in quiescent cells and half were involved in mitochondrial function, consistent with the 6-fold increase in respiration observed in quiescent cells and the relative absence of Cit1p:GFP in non-quiescent cells. Finally, examination of quiescent-cell specific, GFP-fusion proteins revealed symmetry in protein accumulation in dividing quiescent and non-quiescent cells after glucose exhaustion, leading to a new model for the differentiation of these cells.