The hybridization of human DNA with three non-cross-hybridizing monomers (68 bp in length) of the heterochromatic Sau3 A family of DNA repeats, indicates the coexistence within a Sau3A-positive genomic block of divergent Sau3 A units as well as of unrelated sequences. To gain some insight into the structure of these human heterochromatic DNA regions, three previously cloned Sau3A-positive genomic fragments (with a total length of approximately 1900 base-pairs (bp)) were sequenced. The analysis of the sequences showed the presence of clustered Sau3 A units with different degrees of divergence and of two DNA regions of approximately 100 bp and 291 bp in length, unrelated to the family of repeats. A consensus sequence derived from the 24 identified Sau3 A monomers presents, among highly variable regions, two less variant regions of 8 bp and 10 bp in length, respectively. The Sau3A-unrelated DNA fragment 291 bp in length, used as a probe on genomic DNA digested with a series of restriction enzymes, defines a “new” family of DNA repeats possessing periodicities for HaeIII ( HaeIII family). Sau3 A and HaeIII repeats display a high degree of linkage in a collection of Sau3A-positive genomic recombinant phages.