Adult stem cells undergo asymmetric cell division to self-renew and give rise to differentiated cells that comprise mature tissue(1). Sister chromatids may be distinguished and segregated nonrandomly in asymmetrically dividing stem cells(2), although the underlying mechanism and the purpose it may serve remain elusive. Here we develop the CO-FISH(chromosome orientation fluorescence in situ hybridization) technique(3) with single-chromosome resolution and show that sister chromatids of X and Y chromosomes, but not autosomes, are segregated nonrandomly during asymmetric divisions of Drosophila male germline stem cells. This provides the first direct evidence, to our knowledge, that two sister chromatids containing identical genetic information can be distinguished and segregated nonrandomly during asymmetric stem-cell divisions. We further show that the centrosome, SUN-KASH nuclear envelope proteins and Dnmt2 (also known as Mt2) are required for nonrandom sister chromatid segregation. Our data indicate that the information on X and Y chromosomes that enables nonrandom segregation is primed during gametogenesis in the parents. Moreover, we show that sister chromatid segregation is randomized in germline stem cell overproliferation and dedifferentiated germline stem cells. We propose that nonrandom sister chromatid segregation may serve to transmit distinct information carried on two sister chromatids to the daughters of asymmetrically dividing stem cells.