To investigate the role of the yeast telomere-, silencing-, and UAS-binding protein RAP1 in telomere position effects, we have characterized two sets of mutant cells: (1) a set of rap1 alleles (termed the rap1t alleles) that produce truncated RAP1 proteins missing the carboxy-terminal 144-165 amino acids; and (2) null mutants of the RIF1 gene, encoding a protein capable of interaction with the carboxyl terminus of RAP1. The data presented here indicate that loss of the carboxyl terminus of RAP1 abolishes position effects at yeast telomeres and diminishes silencing at the HML locus. Elimination of position effects in these cells is associated with increased accessibility to the Escherichia coli dam methylase in vivo. Thus, the carboxy-terminal domain of RAP1 is required for telomere position effects. In contrast, rif1 deletion alleles increase the frequency of repressed cells. Using the rap1t alleles to generate wild-type cells differing only in telomere tract lengths, we also show that telomere position effects are highly sensitive to changes in the size (or structure) of the telomeric tract. Longer poly(G1-3T) tracts can increase the frequency of transcriptional repression at the telomere, suggesting that telomeric poly(G1-3T) tracts play an active role in the formation or stability of subtelomeric transcriptional states.