We have isolated and sequenced 30 independent clones derived from MnlI digestion of purified mouse major (γ) satellite DNA. These clones contained between 0.9 and 1.1 γ monomeric units derived presumably from random chromosomal sources. Individual clones showed a mean deviation from the mouse consensus satellite sequence of 3.9%, with a range of 0.9–9.1%. Cleavage of total mouse LTK cell genomic DNA with three different restriction enzymes ( HindIII, BglII, BamHI) that do not cut within satellite monomers, followed by Southern and pulsed-field gel electrophoretic analyses, showed that the majority of monomers were organized into largely uninterrupted arrays that varied from a minimum of 240 kb to greater than 2000 kb in length. We suggest that the high degree of conservation of the mouse γ-satellite sequences throughout the mouse genome results from frequent recombinational exchange between nonhomologous chromosomes. Further, this same process, facilitated by the all-acrocentric constitution of the typical mouse karyotype, and the extremely long and homologous γ-satellite arrays, may be related to the common occurrence of Robertsonian translocation in mouse.