Abstract In the yeast Saccharomyces cerevisiae, chromosomes terminate with ∼400 bp of a simple repeat poly(TG1-3). Based on the arrangement of subtelomeric X and Y′ repeats, two types of yeast telomeres exist, those with both X and Y′ (Y′ telomeres) and those with only X (X telomeres). Mutations that result in abnormally short or abnormally long poly(TG1-3) tracts have been previously identified. In this study, we investigated telomere length in strains with two classes of mutations, one that resulted in short poly(TG1-3) tracts (tel1) and one that resulted in elongated tracts (pif1, rap1-17, rif1, or rif2). In the tel1 pif1 strain, Y′ telomeres had about the same length as those in tel1 strains and X telomeres had lengths intermediate between those in tel1 and pif1 strains. Strains with either the tel1 rap1-17 or tel1 rif2 genotypes had short tracts for all chromosome ends examined, demonstrating that the telomere elongation characteristic of rap1-17 and rif2 strains is Tel1p-dependent. In strains of the tel1 rif1 or tel1 rif1 rif2 genotypes, telomeres with Y′ repeats had short terminal tracts, whereas most of the X telomeres had long terminal tracts. These results demonstrate that the regulation of telomere length is different for X and Y′ telomeres.