Cellular senescence, the ultimate and irreversible loss of replicative capacity of cells in primary culture, has been a popular model for studying the aging process. However, the replicative life span of human fibroblasts is heterogeneous even in clonal populations, with the fraction of senescent cells increasing at each population doubling, rather than all cells entering senescence simultaneously. Thus, the study of individual cells in a mass culture is of extreme importance to the understanding of replicative senescence. Cell sorting is a method that allows physical separation of cells with different characteristics when measured by flow cytometry. Here, we describe various methods by which cells that reach senescence early can be physically sorted out of a bulk of growing cells, and discuss how different methods can affect the posterior analysis of the sorted populations.