The eukaryotic genome is divided into functional domains defined in part by local differences in chromatin structure and delimited in many cases by boundary elements. TheHMLandHMRloci in the yeastSaccharomyces cerevisiaeare transcriptionally silent chromosome domains. Each locus is bracketed by two cis-acting sequences, designated E and I, that serve to establish and maintain repression of genes within each locus. We show that repression atHMLis uniformly high between E and I but decreases sharply beyond I. The region of repression atHMLgenerally correlates with the domain of histone hypoacetylation. Despite the sharp definition of the boundaries ofHML, no sequence capable of blocking the spread of heterochromatin resides in the sequences flankingHML. We find, though, that inverting the orientation of I increases silencing outside ofHMLwhile weakening silencing withinHML. These results indicate that theHMLI silencer establishes a boundary between active and inactive chromatin atHML, but does so by organizing inactive chromatin in only one direction. This represents a different mechanism for delimiting the boundaries of a eukaryotic chromosome domain.