Telomere length in the yeast Saccharomyces cerevisiae is under stringent genetic control such that a narrow length distribution of TG₁₋₃ repeats is observed. Previous studies have shown that Rap1p, which binds to the double-stranded telomeric repeats, plays a role in regulating repeat length: point mutations in the Rap 1 p C-terminus often result in a higher average telomere length and deletion of this region causes extreme telomere elongation. We have investigated further the role of Rap1p in this process. Our results suggest that telomere length is regulated by a negative feedback mechanism that can sense the number of Rap1p molecules bound at the chromosome end. This length regulatory mechanism requires two other proteins, Rif1p and Rif2p, that interact with each other and with the Rap1p C-terminus. Although the same C-terminal domain of Rap1p is also involved in the initiation of telomere position effect (telomeric transcriptional silencing), this Rap1p function appears to be separate from, and indeed antagonistic to, its role in telomere length regulation.