Chromosome ends in the lower eukaryotes terminate in variable numbers of tandem, simple DNA repeats. We tested predictions of a model in which these telomeric repeats provide a substrate for the addition of more repeats by a terminal transferase-like mechanism that, in concert with DNA polymerase and primase, effectively counterbalances the loss of DNA due to degradation or incomplete replication. For individual chromosome ends in yeast, the mean length of any given telomere was shown to vary between different clonal populations of the same strain and to be determined by the initial length of that telomere in the single cell giving rise to the clone. This type of variation was independent of the major yeast recombination pathway. The length heterogeneity at each telomeric end increased with additional rounds of cell division or DNA replication. Lengths of individual telomeres within a single clone varied independently of each other. Thus, this clonal variability is distinct from genetic regulation of chromosome length, which acts on all chromosome ends coordinately. These in vivo phenomena suggest that lengthening and shortening activities act on yeast telomeres during each round of replication.