The chromosomes of the yeast Saccharomyces cerevisiae terminate with sequences that have the form poly(C1-3-A). In this paper, we show that within an individual yeast strain all chromosomes end with tracts of poly(C1-3-A) of similar lengths; however, different strains can have tracts that vary in length by a factor of two. By a genetic analysis, we demonstrate that yeast cells have a mechanism that allows them to change rapidly the length of their chromosomes by altering the length of the poly(C1-3-A) tract.